The International Agency for Research on Cancer has coordinated a worldwide study of childhood cancer incidence, with data from over 50 countries. We present here the results on renal tumours. Wilms' tumour was the most common malignant kidney tumour in all regions. It is sometimes considered to be an 'index cancer of childhood' but it is clear from the present study that there is at least a threefold difference in incidence between the age-standardised annual rates of over 10 per million in the Black populations in the United States and Nigeria and those of around three per million in several East Asian populations. In White Caucasian populations, Wilms' tumour had an annual incidence of 6-9 per million, accounting for 5-7% of all childhood cancer. It was almost everywhere equally common in boys and girls, but the sex ratio in East Asia was M/F = 1.4:1. Age distributions were similar among White Caucasian and Black populations, with the peak incidence in the second year of life. In East Asia, however, 25-40% of the total incidence occurred in infants aged under 1 year, compared with around 15% in many Western series. Other studies have shown that, in the United States, Wilms' tumour has a lower incidence among Asian children than among Whites or Blacks and tends to occur at a younger age. The variation in patterns of incidence of Wilms' tumour along ethnic rather than geographical lines suggests that genetic predisposition is important in its aetiology. Renal carcinoma in childhood is rare throughout the world, with little sign of international variation. It accounted for a higher proportion of childhood renal tumours in East Asia but this was attributable to the lower incidence of Wilms' tumour in that region.