Potential predictors of toenail selenium levels were studied in 1211 men and 1248 women aged 55-69 years. These subjects were randomly selected cohort members without prevalent cancer (other than skin) participating in a prospective study on diet and cancer in the Netherlands. Information on the considered potential predictors (gender, age, smoking, intake of dietary selenium and alcohol, Quetelet index) was collected together with toenail specimens in 1986. The average toenail selenium concentration was significantly (P < 0.001) lower in men than in women: 0.547 +/- 0.126 microgram/g (mean +/- SD) and 0.575 +/- 0.109 microgram/g, respectively. The gender difference remained significant after adjustment for the other variables in multiple regression analyses. Age was not associated with toenail selenium levels in men or women. An inverse association was observed with current smoking but not with past smoking. The average toenail selenium values for male current smokers were 0.513 +/- 0.106 microgram/g (mean +/- SD) versus 0.571 +/- 0.133 microgram/g for male never- or ex-smokers (P < 0.001). For women these values were 0.548 +/- 0.101 and 0.581 +/- 0.109 microgram/g, respectively (P < 0.001). Dietary selenium intake was positively associated with toenail selenium levels in multivariate analyses (P < 0.001), but the association was weak (partial r = 0.09). Alcohol intake and Quetelet index were not significant independent predictors of toenail selenium. The observed associations had similar directions in men and women but were stronger in men.