About 20 minutes into the episode, there was a scene in which Pikachu stops some vaccine missiles with its Thunderbolt attack, resulting in a huge explosion that flashed red and blue lights. Although there were similar parts in the episode with red and blue flashes, an anime technique called "paka paka" made this scene extremely intense, for these flashes were extremely bright strobe lights, with blinks at a rate of about 12 Hz for approximately 4 seconds in almost fullscreen, and then for 2 seconds outright fullscreen. At this point, viewers started to complain of blurred vision, headaches, dizziness and nausea. A few people even had seizures, blindness, convulsions and lost consciousness. Japan's Fire Defense Agency reported that a total of 685 viewers (310 boys, 375 girls) were taken to hospitals by ambulances. Although many victims recovered during the ambulance trip, more than 150 of them were admitted to hospitals. Two people remained hospitalized for over 3 weeks. Some other people had seizures when parts of the scene were rebroadcast during news reports on the seizures. Most of them were not diagnosed with photosensitive epilepsy. Later studies showed that 5-10.4% of Pokémon viewers had mild symptoms that did not need hospital treatment. 120,000 children reported mild symptoms of illness, however their symptoms more closely resembled mass hysteria than a grand mal seizure. A study following 103 patients over three years after the event found that most of them had no further seizures.
Scientists believe that the flashing lights triggered photosensitive seizures in which visual stimuli such as flashing lights can cause altered consciousness. Although approximately 1 in 4,000 people (0.5–0.8% of children between 5–13 years old) are susceptible to these types of seizures, the number of people affected by this Pokémon episode was unprecedented.
A Pokémon website, Pokémon Press Battle, made the following observation:
Before the beginning of the reairing, was shown. Broadcast in Japan on April 11, 1998 and April 16, 1998, a woman named Miyuki Yadama went over the circumstances of the program format and the on-screen advisories at the beginning of animated programs.
Despite the negative press netted by the incident, this was, for many people in the United States, Europe and Australia, the first exposure to Pokémon, which would become a major franchise in those areas.
Many Japanese television broadcasters and medical officials got together to find ways to make sure this never happened again. They established a series of guidelines for future animated programs, including:
Japanese broadcasters also began broadcasting an on-screen advisory at the beginning of animated programs. Some example warnings:「テレビを見る時は部屋を明るくして離れて見て下さい」
The episode itself has never been broadcast again in any country. It was reportedly not sent to 4Kids Entertainment for translation, though voice actor Maddie Blaustein claims that the episode was dubbed and altered to slow down the flashing, but never broadcast. The Japanese master copy of the episode is rumored to have been destroyed. In an effort to put the event out of the public's minds, the anime has not featured Porygon in any subsequent episodes (though it did appear briefly at the beginning of the fifth Pokémon movie). Its second-generation evolution, Porygon2, is the only second-generation Pokémon to never make an appearance in the anime.
There was a reference to Dennō Senshi Porygon in an episode of The Simpsons, entitled "Thirty Minutes over Tokyo". Bart is seen watching a cartoon featuring robots with flashing eye lasers, and asks "Isn't this that show that causes seizures?" The flashing eyes proceed to give him a seizure moments later. Soon everyone in the room is having a seizure, except Homer, who comes to the room, sees everyone having seizures and just plays along with them. The show is revealed to be Battle Seizure Robots. The show interrupts to the commercial and everyone stops having the seizures, but after the show goes back everyone, including Homer, this time having real seizures, falls to the floor having seizures again.
In the pilot episode of Drawn Together, Ling-Ling, who is a parody of Pikachu, states that his goal in the Drawn Together house is to "destroy all, and give children seizure". There follows a scene with flashing lights, a direct reference to this episode.
In So Yesterday, a novel by Scott Westerfeld, this episode is mentioned and shown to one of the characters. The flashing red light that caused the seizure is also used in the story telling elements.
In the rock band Dream Theater's concert DVD, "Live Scenes From New York", drummer Mike Portnoy references the seizures in the DVD's commentary during a sequence in which there are many flashing strobe lights.
In March 29, 1997, a 25th episode of an anime called YAT Anshin! Luxury Space Tour had a similar incident when reportedly four children were taken to hospitals by ambulance after watching a scene with rapidly flashing red and white colors. After the "Pokémon Shock" incident it was noted that the YAT Anshin! incident was similar. Ironically, later broadcasts of this episode were edited so the flashes weren't so fast, unlike the Pokemon episode, which was completly banned.