1. During the past twenty-five years there have been admitted to this orthopaedic service twelve children or adolescents having a close relative who previously or subsequently developed slipped upper femoral epiphysis. This represents an incidence of approximately 7 per cent.

2. There is some evidence that the incidence is considerably higher.

3. In addition to those with close relations also with slipped epiphysis, two patients had parents with osteoarthritis of the hip.

4. I believe, therefore, that in slipped upper femoral epiphysis there is evidence of a genetic defect. This is probably due to a recessive gene of low penetrance. The frequency in this region is high because the north-east of Scotland has very definite geographical boundaries and the rural, agricultural population, from which the majority of these cases were drawn, has formed until recently a stable community likely to show a greater than average incidence.