The antiviral effect of 5% idoxuridine in dimethyl sulphoxide intermittently applied and of 40% idoxuridine in dimethyl sulphoxide continuously applied for four days to the lesions in patients with zoster of recent onset was studied in two double-blind controlled trials. Most, but not all, of the patients receiving intermittent active treatment had pain for a short period only. The effect of continuous treatment was striking: pain had disappeared within nine days in all the patients and healing was accelerated. The results were statistically significant.