The average annual incidence of diabetes among 8,688 adult men followed up for five years was 8.0/1,000 with Asian, African and Israeli-born having higher rates than European-born. Multivariate analysis of the findings suggested the following: the most significant variables associated with the development of diabetes are overweight and peripheral vascular disease; the high incidence of diabetes in immigrants from Asia and Africa might be an example of Neel's "thrifty genotype" or failure of adaptation to relatively rapid environmental changes; serum cholesterol level, blood pressure, uric acid level, and education were important also; and the probability of developing diabetes within five years rises from 17/1,000 (when the major variables are low or absent) to 450/1,000 (when they are high and present). This has important clinical implications.