Five hundred thirty-two cases of slipped capital femoral epiphysis (physiolysis colli femoris--PCF) treated at three orthopedic departments in southern Sweden between 1910 and 1982 were used for epidemiologic studies. Three hundred twenty-five cases came from a well-defined area and were used for incidence analyses. During the whole period of investigation, the disease was more common in men than in women. The difference was more pronounced in the earlier years of the investigation and among patients living in the country compared with patients living in the city. The mean age at onset of slipping has decreased in men from 16.0 to 12.7 years and has decreased in women from 12.6 to 11.8 years since the beginning of the century. The left hip is affected more often than is the right, especially in men, but during the past decades there has been a tendency toward equalization. Bilateral slipping was evident in 25.4% of the men and in 17.7% of the women. Men living outside the city were at higher risk for bilateral involvement than were men living in the city. In women, the situation was the opposite. The incidence has followed a periodic pattern with peaks approximately every 20th year. The mean incidence (number of cases/10,000 living born) during the period of growth was 6.1 in men and 3.0 in women. The maximal risk is supposed to be 25.7 in men and 20.5 in women. Men living in the country have always been at higher risk compared with men living in the city. Since the fifties, the incidence in women has also been higher in those living in the country. In women, the incidence was significantly higher between May and August. No seasonal variations were seen in men.