Genital herpes simplex virus (HSV) infection, a sexually transmitted disease (STD), is the commonest cause of ulcerative genital infections among the young and adult population. The significant association of genital ulceration and transmission of human immunodeficiency virus (HIV) has been shown in many studies. To explore the potential efficacy of topical treatment of genital herpes with penciclovir cream, a randomized, double-blind, multicentre, acyclovir-controlled Phase II clinical trial of penciclovir 1% cream 5 times daily up to 7 days for suppression of genital herpes was conducted in China. A total of 205 patients aged 20-59 years (mean age 36.0+/-8.8 years for acyclovir and 34.8+/-8.4 years for penciclovir) with a clinical diagnosis of genital herpes were randomly allocated to one of the 2 parallel treatment groups and used for analysis. Clinical assessment were made before treatment and followed up at every visit during the study. Our results show that there was an encouraging improvement simultaneously in the 2 groups although no significant differences in clinical efficacy with respect to clinical cure rate, and times to healing, resolution of all symptoms, absence of blisters, cessation of new blisters, crusting, and loss of crust between penciclovir and acyclovir groups in terms of primary, non-primary and total patients were found. However a significantly shorter time to crusting was found in primary penciclovir group when compared with primary acyclovir group. Adverse experience was generally infrequent and mild, and was comparable in the 2 treatment groups. Based on these preliminary clinical findings, further evaluation of penciclovir 3% cream for topical treatment of genital herpes is planned.