A study was performed on the effect of increased selenium intake on the serum selenium level of 212 children with juvenile rheumatoid arthritis (JRA) and of 214 controls in a follow-up study during the Finnish nationwide selenium fertilization programme, which was started in 1984. The mean serum level of selenium increased from 0.90 mumol l-1 in 1985 to 1.56 mumol l-1 in 1990 in the children with JRA. The corresponding selenium levels in the controls were 0.87 mumol l-1 in 1985 and 1.33 mumol l-1 in 1990. Boys had slightly higher selenium levels in all the age groups among the patients and the controls. During the entire 10-year observation period, the patients with JRA had slightly higher mean levels of selenium than the controls. The age of the children did not have any significant effect on the selenium level in either group. The present study shows that the main factor affecting the serum level of selenium was the dietary intake of selenium both in patients and in healthy controls.