Vitamin A supplementation of populations of vitamin A-deficient preschool-age children has been shown to reduce childhood mortality, but the primary preventive effects of such supplements on childhood infectious diseases have not been carefully evaluated. We conducted an individually randomized, placebo-controlled, double-masked trial among 1,407 Indonesian preschool-age children, to measure the effects of high dose vitamin A on acute respiratory and diarrheal illnesses. Signs and symptoms of morbidity were monitored using every other day home surveillance by trained interviewers. High dose vitamin A supplements increased the incidence of acute respiratory illnesses (ARI) by 8%, and acute lower respiratory illnesses (ALRI) by 39%. These detrimental effects on acute lower respiratory illnesses were most marked in children with adequate nutritional status (rate ratio 1.83, 95% confidence interval 1.257-2.669). In contrast, vitamin A tended to be protective of ALRI in chronically malnourished children (rate ratio 0.71, 95% confidence interval 0.375-1.331). There was no overall effect of high-dose vitamin A supplements on the incidence of diarrheal disease (rate ratio 1.06, 95% confidence interval 0.920-1.225). However, we found a significant interaction between supplementation and age: vitamin A increased the incidence of diarrhea in children < 30 mo of age, but tended to reduce the incidence in older children. The finding of a significant adverse effect of vitamin A supplements in adequately nourished children highlights the need to review the criteria for selecting populations of preschool-age children for vitamin A supplementation.