A nationwide study of Perthes’ disease in Norway was undertaken over a five-year period from January 1996. There were 425 patients registered, which represents a mean annual incidence of 9.2 per 100 000 in subjects under 15 years of age, and an occurrence rate of 1:714 for the country as a whole. There were marked regional variations. The lowest incidence was found in the northern region (5.4 per 100 000 per year) and the highest in the central and western regions (10.8 and 11.3 per 100 000 per year, respectively). There was a trend towards a higher incidence in urban (9.5 per 100 000 per year) compared with rural areas (8.9 per 100 000 per year). The mean age at onset was 5.8 years (1.3 to 15.2) and the male:female ratio was 3.3:1.

We compared 402 patients with a matched control group of non-affected children (n = 1 025 952) from the Norwegian Medical Birth Registry and analysed maternal data (age at delivery, parity, duration of pregnancy), birth length and weight, birth presentation, head circumference, ponderal index and the presence of congenital anomalies. Children with Perthes’ disease were significantly shorter at birth and had an increased frequency of congenital anomalies.

Applying Sartwell’s log-normal model of incubation periods to the distribution of age at onset of Perthes’ disease showed a good fit to the log-normal curve. Our findings point toward a single cause, either genetic or environmental, acting prenatally in the aetiology of Perthes’ disease.