The prevalence of diabetes mellitus in Israel has been described in various populations and ranges from 4.1% to 8.9%. However, the large immigration wave in the late 1980s and early 1990s could have affected the previous estimates. Moreover, data from the United states and other countries report a significant proportion of undiagnosed diabetes. The main purpose of this study was to estimate the prevalence and the ratio of undiagnosed/diagnosed diabetes in a large sample of workers. The study involved interviews, anthropometric measurements at the work site, and blood glucose determinations (casual capillary, fasting, and when necessary, 2 h after a 75-gram oral glucose load). Diabetes and impaired glucose tolerance (IGT) were diagnosed according to WHO criteria. Five thousand four hundred sixteen workers participated in the study. Three hundred ten reported to be known diabetics (5.7%), 35 (0.7%) were diagnosed as diabetic during the survey, and an additional 93 (1.8%) were classified as IGT. The ratio undiagnosed/diagnosed diabetes was therefore 1:10. Extrapolation of these results produced an estimate of diabetes prevalence in Israel of 10% among people over age 40 years. It seems that in the Israeli health care system, based on universal health insurance and easy access to primary care, the proportion of undiagnosed diabetes is lower than that described in other countries.