Using a survey questionnaire design, we investigated the incidence, site, and nature of jogging injuries among all participants of a popular 16 km race. The response rate was 83.6%. Of 4,358 male joggers, 45.8% had sustained jogging injuries during the 1 year study period, 14.2% had required medical care, and 2.3% had missed work because of jogging injuries. Occurrence of jogging injuries was independently associated with higher weekly mileage (P less than 0.001), history of previous running injuries (P less than 0.001), and competitive training motivation (P = 0.03). Higher mileage was also associated with more frequent medical consultations due entirely to jogging-related injuries. In 33 to 44 year olds (N = 1,757), the number of years of running was inversely related to incidence of injuries (P = 0.02). Injuries were not significantly related to race running speed, training surface, characteristics of running shoes, or relative weight. Achillodynia and calf muscle symptoms were the two most common overuse injuries and occurred significantly more often among older runners with increased weekly mileage. We conclude that jogging injuries are frequent, that the number of firmly established etiologic factors is low, and that, in recommending jogging, moderation should be the watchword.