BACKGROUND: Low vitamin D status has been suggested as a risk factor for type 2 diabetes. Although the epidemiologic evidence is scarce, 2 recent studies have suggested an association. The present study investigated the relation of serum vitamin D with type 2 diabetes incidence using pooled data from these 2 cohorts. METHODS: Two nested case-control studies, collected by the Finnish Mobile Clinic in 1973-1980, were pooled for analysis. The study populations consisted of men and women aged 40-74 years and free of diabetes at baseline. During a follow-up period of 22 years, 412 incident type 2 diabetes cases occurred, and 986 controls were selected by individual matching. Serum vitamin D (serum 25(OH)D) was determined from frozen samples, stored at baseline. Pooled estimates of the relationship between serum vitamin D concentration and type 2 diabetes incidence were calculated. RESULTS: Men had higher serum vitamin D concentrations than women and showed a reduced risk of type 2 diabetes in their highest vitamin D quartile. The relative odds between the highest and lowest quartiles was 0.28 (95% confidence interval = 0.10-0.81) in men and 1.14 (0.60-2.17) in women after adjustment for smoking, body mass index, physical activity, and education. CONCLUSIONS: The results support the hypothesis that high vitamin D status provides protection against type 2 diabetes. Residual confounding may contribute to this association.