Haloperidol treatment for stuttering was examined in a double-blind cross-over study of 26 adult volunteers with long-standing stuttering. Most had had unsuccessful speech and drug therapy. During the three-month study, their speech and stuttering patterns were repeatedly evaluated from videotaped readings of a standard passage and during spontaneous speaking. Of the 18 patients completing the trial 11 were significantly more improved on haloperidol than on placebo, three benefited equally from both, and four were unchanged. Improvement over placebo was indicated by fewer dysfluencies, increased speed of speaking, and reduced secondary 'struggle’ phenomena. Side effects were common on a dosage of 3 mg haloperidol daily. Poor concentration, akathisia and dystonic movements caused 8 patients to discontinue the trial despite significant improvement in 5 of them. Although 'statistically significant’ improvement occurred in most patients on haloperidol, the ‘clinical significance’ of this form of therapy will be limited by the partial response, the need for continuous medication, the side effects of haloperidol and the attitude of stutterers to this type of therapy.