To describe the incidence and type of injury and illness occurring during an expedition-length adventure race and identify those resulting in withdrawal from the event, a prospective cohort study was conducted of the injuries and illness treated during the Subaru Primal Quest Expedition Adventure Race trade mark held in Colorado July 7-16, 2002. All racers, support crewmembers, and race staff were eligible to participate in the study. When a member of the study group received medical care due to an injury or illness, the encounter was recorded on a Medical Encounter Form. If an injury or illness resulted in withdrawal from the race, this was also recorded. Information from the Medical Encounter Forms was used to generate the Medical Log. There were 671 individuals eligible to participate in the study. A total of 243 medical encounters and 302 distinct injuries and illnesses were recorded. There were 179 (59%) injuries and 123 (41%) illnesses. Skin and soft tissue injuries and illness were the most frequent (48%), with blisters on the feet representing the single most common (32.8%). Second was respiratory illness (18.2%), including upper respiratory infection, bronchitis and reactive airway disease-asthma. Respiratory illness was the most common medical reason for withdrawal from the event. Injuries accounted for almost 60% of all injury and illness yet they contributed to less than 15% of the medical withdrawals from the race. Blisters accounted for almost one-third of all conditions treated. Providers of medical support for expedition-length adventure races should be prepared to treat a wide variety of injury and illness.