The effectiveness of a megavitamin regimen utilizing a two-stage trial in 41 subjects with attention deficit disorders was studied. Stage 1 was a 3-month clinical trial of vitamins (daily maximum: 3 g of niacinamide and ascorbic acid, 1.2 g of calcium pantothenate, and 0.6 g of pyridoxine). State 2 consisted of four, 6-week, double-blind repeated crossover periods. Twenty-nine per cent of the subjects showed significant behavior improvement during stage 1, and these subjects were used in the double-blind crossover phase of the study to evaluate megavitamin therapy. Using analysis of variance methods for crossover studies, there was no significant difference (P greater than .05) in most behavior scores between children receiving vitamin and those receiving placebo during stage 2. Children exhibited 25% more disruptive classroom behavior when treated with vitamins v placebo (P less than .01). There was no significant difference in serum pyridoxine and ascorbic acid levels between subjects and control subjects. Forty-two per cent of subjects exceeded the upper limits of serum transaminase levels while receiving vitamins. It is concluded that megavitamins are ineffective in the management of attention deficit disorders and should not be utilized because of their potential hepatotoxicity.