This study examined the extent and patterns of use of acyclovir in a health maintenance organization (HMO) population. The development and implementation of a system to survey acyclovir users for possible acute adverse effects is also described. Acyclovir users were members of Kaiser Permanente (KP), Northwest Region, who received one or more prescriptions for acyclovir from an automated outpatient prescription system over a two-year period (1986 and 1987). KP members with genital herpes were identified from the automated Outpatient Utilization System, which abstracts medical record data from a random sample of about two percent of KP members. Acyclovir users with hospital admissions were identified from the automated KP hospital discharge abstract system. During the two years, there were 2940 users who received a total of 6182 prescriptions for acyclovir; 47 percent used oral acyclovir and 56 percent of prescriptions were for the oral dosage form. Females received two-thirds of the prescriptions, and people 15 through 45 years of age received 80 percent of the prescriptions. The estimated incidence density of oral acyclovir usage was 2.3/1000 KP members. Most oral acyclovir prescriptions were for the treatment of genital herpes. More than 60 percent of the prescriptions appeared to be for first treatment, more than 30 percent for repeat treatment, and about 6 percent for continuing treatment. Hospitalizations of oral acyclovir users appeared to be unrelated to the use of the drug. © 1990, SAGE Publications. All rights reserved.