The efficacy and safety of penciclovir (PCV) for the treatment of herpes simplex virus (HSV) infections in immunocompromised (IC) patients were studied in a double-blind, acyclovir (ACV)-controlled, multicenter study. A total of 342 patients with mucocutaneous HSV infections received 5 mg of PCV per kg every 12 or 8 h (q12h or q8h) or 5 mg of ACV per kg q8h, beginning within 72 h of lesion onset and continuing for up to 7 days. The mean age of the patients was 49 years; 94% were white and 52% were female. The main reasons for their IC states were hematologic disorder (63%) and transplant plus hematologic disorder (16%). Clinical and virological assessments were performed daily during the 7-day treatment and then every other day until lesion healing. The primary efficacy parameter addressed new lesion formation. Secondary end points focused on viral shedding, healing, and pain. Approximately 20% of patients in each treatment group developed new lesions during therapy; thus, equivalence with ACV (defined prospectively) was demonstrated for both q12h and q8h PCV regimens. For all three treatment groups, the median time to the cessation of viral shedding was 4 days and the median time to complete healing was 8 days; there were no statistically significant differences in the rates of complete healing or the cessation of viral shedding when the results for PCV q12h and q8h were compared with those for ACV q8h. In addition, there was no statistically significant difference between PCV q12h or q8h, compared with ACV q8h, for the resolution of pain. PCV was well tolerated, with an adverse event profile comparable to that of ACV. In conclusion, PCV q12h is a well-tolerated and effective therapy for mucocutaneous HSV infection in IC patients and offers a reduced frequency of dosing compared with ACV q8h.