We used ultrasonography to study 26 hips with slipped capital femoral epiphyses. In recently slipped epiphyses the ultrasound image revealed a step at the anterior physeal outline (mean 6.4 mm), diminished distance between the anterior acetabular rim and the femoral metaphysis (mean 4.3 mm) and an effusion. As metaphyseal remodelling progressed the physeal step decreased. The femoral neck appeared straighter in hips which had been symptomatic for longer than three weeks. It was possible to measure posterior epiphyseal displacement without projectional errors and the method was accurate in diagnosing minimal slip and in staging displacement. The suggested criteria are, less than 7 mm for a mild slip, 7 to 11 mm for a moderate slip and more than 11 mm for a severe slip. We recommend ultrasonography for the diagnosis, staging and follow-up management of slipped upper femoral epiphysis.