During the period of 1976 to 1981, a total of 412 young athletes contacted the Turku Sports Medical Research Unit's (TSMRU) Outpatient Sports Clinic with 586 complaints. These records included 68 athletes with Osgood-Schlatter's disease (OSD), who were initially prescribed an average of 2 months' rest from any physical activity causing pain. Symptoms of tibial tuberosity pain occurred first at the average age of 13.1 years. According to the retrospective questionnaire, the pain caused complete cessation of training for an average of 3.2 months, and the disease interfered with fully effective training for an average of 7.3 months. According to a retrospective questionnaire given to 389 students (191 girls and 198 boys), 50 (12.9%) had suffered from OSD. Nearly one-half of the students, 193 (49.6%), had been active in sports at the age of 13, and 41 (21.2%) of them had suffered from OSD. In those students who were not active in sports, the incidence was only nine (4.5%; P less than 0.001). The incidence (32%) was higher in the siblings of the OSD patients of TSMRU who were active in sports than in the corresponding student group (21.2%). In a group of 22 patients from the TSMRU who had suffered from Sever's disease (calcaneal apophysitis), the incidence of OSD (68%) was significantly higher than in students who were active in sports (P less than 0.001).