Increased serum insulin is related to abdominal obesity and high blood pressure in affluent societies where insulin, weight, and blood pressure typically increase with age. The increased insulin level has been thought to reflect insulin resistance, a well-known associated factor in the metabolic syndrome. In most nonwesternized populations, body weight and blood pressure do not increase with age and abdominal obesity is absent. However, it is not known whether serum insulin likewise does not increase with age in nonwesternized societies. Fasting levels of serum insulin were measured cross-sectionally in 164 subsistence horticulturalists aged 20 to 86 years in the tropical island of Kitava, Trobriand Islands, Papua New Guinea, and in 472 randomly selected Swedish controls aged 25 to 74 years from the Northern Sweden WHO Monitoring Trends and Determinants in Cardiovascular Diseases (MONICA) Study. In Kitava, the intake of Western food is negligible and stroke and ischemic heart disease are absent or rare. The body mass index (BMI) and diastolic blood pressure are low in Kitavans. The main outcome measures in this study were the means, distributions, and age relations of serum insulin in males and females of the two populations. Serum fasting insulin levels were lower in Kitava than in Sweden for all ages (P < .001). For example, the mean insulin concentration in 50- to 74-year-old Kitavans was only 50% of that in Swedish subjects. Furthermore, serum insulin decreased with age in Kitava, while it increased in Sweden in subjects over 50 years of age. Moreover, the age, BMI, and, in females, waist circumference predicted Kitavan insulin levels at age 50 to 74 years remarkably well when applied to multiple linear regression equations defined to predict the levels in Sweden. The low serum insulin that decreases with age in Kitavans adds to the evidence that a Western lifestyle is a primary cause of insulin resistance. Low serum insulin may partly explain the low prevalence of cardiovascular disease in Kitavans and probably relates to their marked leanness.