The objective of this paper was to evaluate the efficacy of diphenhydramine hidrochloride (DPH) in dystonic patients. In 1995, Truong et al reported encouraging results in five patients with idiopathic torsion dystonia (ITD) treated with DPH, an H1 antagonist with sedative and anticholinergic properties. Five patients with generalized ITD, one with secondary generalized dystonia and one with idiopathic segmental dystonia were included in the prospective study. Initially the response to intravenous administration of DPH versus placebo in two sessions a week apart was evaluated. Two weeks later all patients started oral DPH in increasing doses (range 100-300 mg, mean 164 mg). The degree of dystonia was determined by a modified University of Columbia Scale evaluating the baseline score, after placebo and DPH I.V. administration then at one and six months after starting oral treatment. The results were analyzed by Friedman's test for repeated measurements. On comparing scores for baseline severity, I.V. placebo and I.V. DPH presented a highly significant correlation (12.09; p = 0.00) as well as comparing baseline score with oral DPH at one and 6 months, treatment (12.78; p = 0.00). Functional score results were 9.5 p=0.01 and 8.4 p=0.02 at one and 6 months respectively. The most common side effects were somnolence and dizziness. It can be concluded that DPH proved effective in our patients with mild to moderate adverse effects not requiring drug withdrawal in any case. However, I.V. challenge was unable to predict the long-term response to oral medication perhaps due to the limited number of cases.