The purpose of this study was to provide additional information on the clinical course and viral replication kinetics of herpes simplex virus (HSV) in recurrent herpes labialis. Data were obtained on 20 subjects who were followed for five days from the first day of the lesion. HSV was isolated in 17 subjects (85%); 75% were positive on day 1 of the lesion. Median HSV titer on day 1 was 1.7 x 10(3)/0.2ml. Isolation rates and titers dropped sharply and virus could no longer be detected by day 4. The clinical course both in terms of the frequency and the severity of the symptoms paralleled the kinetics of viral replication. Thermography delineated location of subsequent lesions in three early symptomatic patients who were studied at the time when no visible lesions were observed. These data are felt to provide useful background information for future studies on the efficacy of topical antiviral agents.