Among school-aged children, unintentional injuries are the leading cause of morbidity and mortality. We began this prospective study in a sample of nine schools within the Boulder Valley School District (Colorado) during the 1988-1989 school year in an effort to explore the etiology of school-related injuries and to provide information relevant to their prevention. During the study period, 509 injuries were reported among a population of 5,518 students, yielding an incidence of 9.22 per 100 students. Boys were nearly one and one-half times more likely to have sustained a school-related injury than girls (risk ratio (RR) = 1.41; 95% confidence intervals [CIs] 1.18, 1.68). We found a significant difference among injury rates by school level (P less than .001). Middle/junior high students had the highest rate, followed closely by elementary students and distantly by high school students. Sports activities accounted for the largest percentage of school injuries (53%), and the percentage of sports-related injuries increased with increasing grade level. Analysis of injury rates by school location revealed that high school students were most frequently injured in the gym (1.52 per 100), middle/junior high students on the athletic field (4.26 per 100), and elementary students on the playground (6.12 per 100). Using injury location as the focal point, we examined relationships among the variables sport/activity, body site, and nature of injury. We found that a large percentage of injuries sustained on the athletic field or in the gym were similar and affected similar body sites, whereas playground injuries differed in their nature and in body site affected.(ABSTRACT TRUNCATED AT 250 WORDS)