OBJECTIVE - A survey of noncommunicable diseases (NCD) in the Pacific island population of Western Samoa in 1978 (n = 1.206) documented a relatively high prevalence of non-insulin-dependent diabetes mellitus (NIDDM) and obesity. A follow-up survey was performed in 1991 (n = 1,776) to assess changes in NCD prevalence and risk factor distribution over 13 years. RESEARCH DESIGN AND METHODS - In both surveys, the same representative villages from one urban and two rural areas were studied, and the survey procedure included an oral glucose tolerance test, anthropometric and blood pressure measurements, and physical activity assessment (1991 only). RESULTS - The age-standardized prevalence of NIDDM in 1991 was 9.5 and 13.4% in Apia (urban) for men and women, respectively. In Poutasi (rural), 5.3% of men and 5.6% of women had NIDDM, and in Tuasivi (rural) the prevalence was 7.0 and 7.5% for men and women, respectively. Age, body mass index (BMI), waist-to- hip circumference ratio, physical inactivity, and family history of diabetes all showed independent association with NIDDM and impaired glucose tolerance. Living in Apia (compared with Poutasi) was also associated with NIDDM. Between 1978 and 1991, the age-standardized prevalence of NIDDM in Apia increased from 8.1 to 9.5% in men and 8.2 to 13.4% in women. In Poutasi, a dramatic increase occurred in prevalence from 0.1 to 5.3% in men, but little change in women was noted (5.4 to 5.6%). In Tuasivi, the increases were 2.3 to 7.0% in men and 4.4 to 7.5% in women. In combined survey areas, increases were observed in the age-standardized prevalence of obesity and mean levels of total cholesterol, fasting triglycerides, and uric acid between surveys as well as a reduction in the prevalence of smoking. CONCLUSIONS - This is the first study using standardized methods to show a dramatic increase in the prevalence of NIDDM in a developing Pacific Island population, and it indicates the importance of maintaining and expanding preventive programs for NIDDM and related lifestyle diseases in these populations.