The records of 224 children who had a slipped capital femoral epiphysis and who had no underlying metabolic or endocrine disorder were studied retrospectively to investigate the epidemiology of bilateral slipped capital femoral epiphysis. Eighty-two (37 per cent) of the 224 children (fifty-one boys and thirty-one girls) had a bilateral slip. Sixty-four of these children were black and eighteen were white. The age at the time of the diagnosis of the first slip was 13 +/- 1.7 years (mean and standard deviation), the duration of the symptoms was 5 +/- 5.0 months, and the angle of the slip was 26 +/- 16 degrees. Obese children were younger at the time of the diagnosis of the first slip (12 +/- 1.6 compared with 13 +/- 1.6 years for the children who were not obese, p = 0.001). The diagnosis of a slipped capital femoral epiphysis was made simultaneously in both hips in forty-one children and sequentially in forty-one children. Compared with the children in whom both hips were diagnosed simultaneously, the children in whom the hips were diagnosed sequentially had had a shorter duration of the symptoms before the diagnosis of the first slip (3 +/- 2.4 compared with 7 +/- 5.9 months, p = 0.0003), were younger at the time of the diagnosis of the first slip (12 +/- 1.9 compared with 13 +/- 1.2 years, p = 0.001), and tended to be more obese (p = 0.025). In 88 per cent of the patients who had sequential slips, the second slip was diagnosed within eighteen months after the diagnosis of the first slip.(ABSTRACT TRUNCATED AT 250 WORDS)