A rising incidence of insulin-dependent diabetes mellitus (IDDM) has been reported in many northern European countries, with a rate equivalent to a doubling time of 20-30 yr in some. North American and Japanese studies report a similar trend, although they are less uniform in their findings. Although the number of genetically susceptible individuals within these populations has increased, the rapidity of the change suggests that environmental factors are responsible. If these could be identified, primary prevention might become possible.