The association between breast cancer and selenium, measured in the diet, plasma, erythrocytes, and toenails, was investigated in a case-control study in the Netherlands during 1985-1987. Dietary selenium intake was assessed by a structured dietary history technique among 133 breast cancer cases and 238 population controls. Absolute and energy-adjusted selenium intake and selenium concentrations in plasma, erythrocytes, and toenails were similar in cases and controls. The multivariate adjusted odds ratios of breast cancer for subjects in the lowest quartile compared with the highest quartile were 1.6 (95% confidence interval (CI) 0.8-3.4) for dietary selenium, 2.0 (95% CI 0.9-4.4) for plasma selenium, 0.9 (95% CI 0.4-1.9) for erythrocyte selenium, and 1.1 (95% CI 0.6-2.1) for toenail selenium. No statistically significant trend was observed in the odds ratios for any of the four indicators of selenium. These results do not suggest a substantial association between selenium and breast cancer for both short- and long-term markers of selenium status.