Study objective:To determine injury and illness patterns and occurrence rates during wilderness recreation. Design: Prospective injury and illness surveillance study. Setting: Wilderness areas throughout the Western hemisphere. Type of participants: All students and instructors on National Outdoor Leadership School courses over a five-year period. Main results: A single fatality occurred, resulting in a death rate of 0.28 per 100,000 person-days of exposure. Injuries occurred at a rate of 2.3 per 1,000 person-days of exposure. Sprains and strains and soft tissue injuries accounted for 80% of the injuries. The illness rate was 1.5 per 1,000 person-days of exposure. Sixty percent of illnesses were due to nonspecific viral illnesses or diarrhea; hygiene appeared to have a significant impact on the incidence of these illnesses. Thirty-nine percent of the injuries and illnesses required evacuation (1.5 per 1,000 person-days of exposure). Conclusion: The injury and illness patterns indicate that wilderness medical efforts should concentrate on wilderness hygiene and management of musculoskeletal injuries and soft tissue wounds. The data also indicate that wilderness activities can be conducted relatively safely, but the decision to participate should be individualized, with an understanding of risks versus benefits.