It is accepted that the prevalence of known diabetes mellitus has increased in some countries, although the available data are incomplete and sometimes anecdotal. In 1974 a sample of the urban population (n = 21,410) in a suburb of Athens (Greece) was evaluated for the prevalence of known diabetes mellitus. The study was repeated 17 years later in the same area under the same conditions and methodology on 12,836 randomly selected inhabitants (both sexes, all ages). Visiting nurses contacted the inhabitants at home and filled in a questionnaire. The results show a clear increase in diabetes prevalence of the population as a whole (standardized rates: 1974: 2.4% (95% confidence intervals 2.2-2.6%) 1990: 3.1 (2.8-3.4%)). Standardization was done on the basis of data obtained from the same census. The changes were statistically significant in the 50-59, 60-69, and 70-79 age groups (1974: 5.87, 10.98, and 11.41%; 1990: 7.70, 13.68, and 19.52%, respectively). Thus, the prevalence of known diabetes mellitus has considerably increased during the last 17 years in a sample of urban Greek population.