This double-blind and placebo-controlled clinical trial in autistic children had three objectives: (a) to replicate earlier findings that haloperidol administration is associated with a significant reduction of behavioral symptoms; (b) to further assess its safety when given on a short-term basis; and (c) to assess whether it has an effect on discrimination learning. Forty-five children, 2.02 to 7.58 years old (M = 4.49), completed this crossover design, with random assignment to treatment sequences. Haloperidol was shown to be a powerful therapeutic agent when administered for 4 weeks and free of side effects; at doses ranging from 0.25 to 4.0 mg/day (M = 0.844), there was a clinically and statistically significant reduction of a variety of symptoms. Under the given conditions, the children failed to learn on either haloperidol or placebo.