In a double-blind trial involving acutely manic patients, clonazepam was compared to placebo, both groups receiving chlorpromazine as needed. The group receiving clonazepam showed significantly more improvement in their manic but not their psychotic symptoms compared to the placebo group. This effect was not primarily related to the sedative effects of clonazepam. Clonazepam tended to reduce the need for phenothiazine medication. Side effects related to sedation were more common to the clonazepam group. These findings are the first to indicate that clonazepam may have a specific antimanic effect over and above that of phenothiazines alone.