In a double-blind crossover study, single doses of d-amphetamine and placebo were administered to 12 patients with severe chronic obsessive-compulsive disorder (OCD). Improvement of obsessional symptoms was significant on clinical ratings and was correlated with improved performance on an attention task. Changes were also significant for self-rated measures of activation and altered reality. The behavior response to amphetamine was not statistically correlated with subsequent improvement during a 6-week clomipramine trial, although the direction of change was the same during both treatments for every patient studied.