OBJECTIVE To examine whether serum levels of alpha-tocopherol, beta-carotene and retinol were associated with risk of non-insulin dependent diabetes mellitus (NIDDM). METHODS The study design was a nested case-control study within a longitudinal population study. Serum levels of antioxidants were determined in 106 incident cases with non-insulin dependent diabetes mellitus detected on follow-up and 201 controls matched for sex, age and study region. RESULTS The incident cases had lower serum alpha-tocopherol and beta-carotene levels than controls. The relative risk between the highest and lowest tertiles of serum alpha-tocopherol was 0.61 (95% confidence interval (CI) 0.32-1.15), between the highest and lowest tertiles of serum beta-carotene 0.45 (CI 0.22-0.92). Although the relative risk of highest compared with lowest tertile of serum alpha-tocopherol was not statistically significant the inverse trend through the tertiles was (P < 0.05). The decreasing risk of diabetes was particularly evident in the elderly, women, nonsmokers and the obese. However, adjustment for serum cholesterol, obesity, smoking and hypertension abolished the associations. The adjusted relative risks in relation to serum alpha-tocopherol and beta-carotene (between highest and lowest tertiles) were 1.25 (CI 0.54-2.90) and 0.94 (CI 0.38-2.32), respectively. No associations were observed relating to serum retinol. CONCLUSIONS High levels of alpha-tocopherol and beta-carotene were found to be associated with decreased risk of non-insulin dependent diabetes mellitus, but the association disappeared after adjustment for cardiovascular risk factors.