In a 2-armed, double-blind, randomized clinical study, the efficacy in the treatment of recurrent herpes labialis of 5% acyclovir in a novel liposomal carrier (ethosome) was evaluated in comparison with that of a commercial 5% acyclovir cream (Zovirax cream) and that of a drug-free vehicle. Data were based on 61 herpetic episodes in 40 subjects. In a crossover arm in which the 2 active preparations were compared, the time to crusting of lesions was significantly shorter (P < .025) with the ethosomal acyclovir (1.8 days) than with the cream (3.5 days). Time to loss of crust was also significantly shorter (4.2 vs 5.9 days; P < .05). In a parallel arm in which all 3 preparations were compared, the time to crusting with the ethosomal acyclovir (1.6 days) was significantly shorter than the time with the acyclovir cream (4.3 days; P < .02) and the time with the drug-free vehicle (4.8 days; P < .005); in this arm, the shorter time to loss of crust for the ethosome (3.5 days), in comparison with the times for the cream (6.4 days) and the drug-free vehicle (6.1 days), did not reach statistical significance. Approximately 30% of all episodes treated with the ethosome were clinically abortive; this compared with 10% of those treated with the cream or the drug-free vehicle. No adverse effects were reported, other than minor burning sensations at the application site that lasted a few seconds after application and were evenly distributed between the investigated preparations. This pilot study suggests the improved clinical efficacy of the new liposomal preparation in comparison with Zovirax cream in the treatment of recurrent herpes labialis.