The effect of increased selenium uptake on serum selenium in diabetic children was investigated during the first 9 yr of the Finnish nationwide selenium fertilization program, which started in 1984. Serum selenium concentrations were followed in 237 diabetic children (mean age 8.1 yr) and 214 controls from 1984 to 1992. The control group consisted of 107 siblings of the diabetics and of 107 other healthy children of corresponding age groups. Selenium was determined by direct electrothermal atomic absorption spectrophotometry. The effect of the increased uptake was seen in both diabetic and in control persons. Before the autumn of 1985, diabetic patients had significantly higher serum selenium levels than their siblings or the other healthy controls. Toward the end of year 1987, this difference had disappeared. After that, serum selenium levels continued to increase until the year 1990. In 1990 the mean selenium serum level of diabetic patients was 1.36 mumol/L and that of controls 1.33 mumol/L. The duration of diabetes did not have any effect on selenium serum levels. Slightly higher serum selenium in new diabetic patients before the start of therapy was explained by the dehydration state. The patients who were younger than 3 yr had slightly lower selenium serum levels when compared with older age groups. This difference was observed, however, only during the first 3 yr of the study. After that, when the selenium intake increased in general, no age-dependent differences were found anymore. There were no significant differences in serum selenium levels between males and females in either diabetic patients or in controls.