To evaluate the effects of megadoses of vitamins with minerals on the cognitive intelligence of children with Down syndrome, a two-group double-blind clinical trial was carried out with 56 school-aged children with Down syndrome. Children were evaluated at baseline, 4 months, and 8 months with a battery of standard psychologic tests, physical examinations, and blood tests. The two groups, which were well-matched is cognitive intelligence and other important subject characteristics at baseline, were not significantly different in intelligence and other test scores at the 4- or 8-month test periods. The particular megadoses of vitamins with minerals used in the study did not produce increased intelligence in the study population.