Substantial differences in the level and patterns of cancer have long been known to exist. Thus, breast cancer mortality in England & Wales in 1908-1912 was ten times higher than in Japan. Today the risk differential is six-fold. The major geographical differences in cancer risk throughout the world are mentioned and the significance of study of changes in cancer risk in migrant populations is emphasised. Thus, while cancer of the large bowel is still relatively uncommon in Japan, the incidence in US Japanese is currently higher than in both US Whites and Blacks. As the Japanese have not changed their genes, it is likely that the higher levels of risk in the US are due to the environment. Within Singapore there are substantial differences in the risk of cancers of the nasopharynx and oesophagus between the various Chinese dialect groups. The information available on ethnic differences in cancer risk in the UK are reviewed. Current analyses are flawed by failure to distinguish between ethnic groups coming from the same continent. The collection of data on ethnic group at the 1991 census and the recently introduced requirement that this also be collected in hospital records will permit direct calculation of incidence and replace anecdote by fact.