This is the first large survey carried out in Iceland to estimate the prevalence and incidence of known and unknown non-insulin-dependent (Type 2) diabetes (NIDDM) among males and females, aged 34-79. The population in this survey was 9128 males and 9759 females born between 1907 and 1935 and examined in the prospective Reykjavik Study 1967-1991. Participants were invited from one to five times during the 24 years. The overall age-standardized prevalence (95% confidence limits) was 2.9% (2.5 to 3.3) for males and 2.1% (1.8 to 2.5) for females, aged 30-79, according to the European standard population. The overall annual age-standardized incidence rate per 100,000 was 377 (303 to 457) for males and 266 (212 to 320) for females, aged 35-74, standardized to the European population. Our study indicates that the prevalence of NIDDM is relatively low compared to other Nordic and western countries, and has not been increasing over the past 20 years. Furthermore, the incidence of NIDDM has not been changing during the past 20 years of follow-up among Icelandic males and females aged 34-79.