While it is a common misconception that they are harmful, crabapples are not poisonous or dangerous to humans when ingested. The worst possible outcome from eating crabapples is a stomach ache from eating too many at one time.
Because of the diminutive size of the fruits they bear, crabapple trees are generally considered decorative trees rather than edible fruit trees. The best way to tell if a crabapple is considered decorative or edible is based on its size. If the fruit is 2 inches or more in diameter, it is considered to have come from an edible fruit tree. Crabapples with a smaller diameter pose no greater risk to humans than their larger counterparts, but they have a very bitter and unappetizing flavor.
Even edible crabapples have a flavor that would not appeal to most people if eaten raw. Crabapples are best enjoyed after they are cooked and sweetened. They are commonly made into jelly, jam, sauces and even pickles.