Roosevelt had originally planned to go on a speaking trip of Argentina and Brazil, followed by a cruise of the Amazon River. Instead, the Brazilian Government suggested that Roosevelt accompany famous Brazilian explorer Cândido Rondon on his exploration of the previously unknown River of Doubt, the headwaters of which had only recently been discovered. Roosevelt, seeking adventure and challenge after his recent defeat for a third term in the White House, agreed. Kermit Roosevelt, Theodore's son, had recently gotten engaged to a socialite named Belle and didn't plan on joining the expedition but did on the insistence of his mother to protect his father. The expedition started in Caracerés, a small town on the Paraguay River, with 15 Brazilian porters (camaradas), the two leaders, Roosevelt's son, and American naturalist George Cherrie. They traveled to Tapirapua, where Rondon had previously discovered the headwaters of the River of Doubt. From Tapirapua, the expedition traveled northwest, through dense forests and then later through the plains on top of the Parecis plateau. They reached the River of Doubt on February 27, 1914. At this point, due to a lack of food supplies, the Expedition split up, with part of the Expedition following the Jiparana river to the Madeira River. The remaining party then started down the River of Doubt.
Almost from the start, the expedition was fraught with problems. Insects and disease such as malaria weighed heavily on just about every member of the expedition, leaving them in a constant state of sickness, festering wounds and high fevers. The heavy dug-out canoes were unsuitable to the constant rapids and were often lost, requiring days to build new ones. The food provisions were ill-conceived forcing the team on starvation diets. Native Indian cannibals (the Cinta Larga) shadowed the expedition and were a constant source of concern - the Indians could have at any time wiped out the expedition and taken their valuable metal tools but luckily they chose to let them pass (future expeditions in the 1920s were not so lucky). One of the camaradas murdered another, while a third was killed in a rapid.
By the time the expedition had made it only about one-quarter of the way down the river, they were physically exhausted and sick from starvation, disease and the constant labour of hauling canoes around rapids. Roosevelt himself was near death as a wounded leg had become infected and the party feared for his life each day. Luckily they came upon "rubber men" or "seringueiros", impoverished rubber-tappers who earned a marginal living from the forest trees driven by the new demand for rubber tires in the United States. The seringueiros helped the team down the rest of the river (less rapid-prone than the upper reaches) and Roosevelt made it home alive to live five more years. Due to the trip, his health never fully recovered.
In 1992 Tweed Roosevelt, with the help of 20 men and women, retraced his great grandfather's journey down the River of Doubt.