All children in Olmsted County, Minnesota, who had a physeal fracture in the 10-year period 1979 through 1988 were identified in this population based study. Children with acute fractures from surrounding areas of Olmsted County and children with subacute, chronic fractures or complications of fractures among referral patients were not included. Eight-hundred fifty children sustained 951 physeal fractures; 561 boys (66%) sustained 637 fractures, and 289 girls (34%) experienced 314 fractures. The male:female ratio was 2:1 and incidence rates were greatest among 11-12 year-old girls and 14-year-old boys. The overall age- and sex-adjusted incidence of physeal fractures was 279.2 per 100,000 person-years (95% confidence interval, 261.4-296.9). The most common site was the phalanges of fingers, which accounted for 37% of all physeal fractures. Salter-Harris type II was the most common type of fracture (54%), but 149 fractures (16%) did not fit into this classification. Therefore, two new, previously unclassified fracture types were added and are reported in detail (see Physeal Fractures: Part 2. Two Previously Unclassified Types, pp. 431-38). This led to a review of existing classifications and creation of a new one (see Physeal Fractures: Part 3. Classification, pp. 439-48).