To assess the incidence of and risk factors for injuries in a group of bicyclists with a well-defined exposure to bicycling, we conducted a prospective study of 1638 recreational bicyclists who rode in the 6-day 339- mile Cycle Across Maryland tour in 1994. The mean age of participants was 39 years (range, 7 to 79), and two-thirds were male. All riders wore helmets. During the tour there were 85 acute traumatic injuries (15.4 per 100,000 person-miles), 76 overuse injuries (13.7 per 100,000 person-miles), and 37 other medical problems (6.7 per 100,000 person-miles). Acute traumatic injuries were associated with a history of racing versus none (relative risk = 2.2, 95% confidence limits = 1.3, 3.7) and with inexperience, no previous Cycle Across Maryland tours versus one or more (relative risk = 1.7, 95% confidence limits = 1.04, 2.8), but not with sex, training, or prior injuries. Inexperience and lack of preride conditioning were risk factors for overuse injuries. The most common overuse injuries and medical problems were knee pain, hand or wrist numbness, foot blisters, insect stings and bites, and heat and dehydration. Study results provide exposure-based incidence rates of bicyclist injuries and suggest overuse injuries may be reduced by increased preride conditioning.