In a double-blind study, we compared the final outcome of 99 Bell's palsy patients treated with either acyclovir-prednisone (53 patients) or placebo-prednisone (46 patients). For patients receiving acyclovir, the dosage was 2,000 mg (400 mg 5 times daily) for 10 days. Electrical tests included electroneurography and the maximal stimulation test. Univariate comparisons of outcome and electrical tests between the two groups were made with chi 2 analysis, Fisher's exact test, and t-tests. The outcome in acyclovir-prednisone-treated patients was superior to that in placebo-prednisone-treated patients. Treatment with acyclovir-prednisone was statistically more effective in returning volitional muscle motion (recovery profile of 10; p = .02) and in preventing partial nerve degeneration (p = .05) than placebo-prednisone treatment. The t-tests indicated that the recovery profile and index means were significantly better for the acyclovir-treated group (recovery profile t = 1.99, p = .051; recovery index t = 2.10, p = .040). We conclude that acyclovir-prednisone is superior to prednisone alone in treating Bell's palsy patients and suggest that herpes simplex is the probable cause of Bell's palsy.