87 patients with the clinical diagnosis of herpes zoster were seen during a one-year period in 8 general practices in Glasgow, the rate per 1 000 practice population being approximately 2.4. Of these, 78 (90%) had serological evidence of active infection with herpes zoster. A history of recurrent herpes simplex was obtained in 25 (32%) of the 78 patients with confirmed herpes zoster; 63 (81%) of these 78 had complement-fixing (CF) antibodies to herpes simplex. Thus, latent herpes simplex infection did not prevent herpes zoster nor modify the severity of herpes zoster. In most of the patients CF antibodt to varicella-zoster was not detected within 5 days from appearance of the rash. After the acute phase of the illness CF antibody titres fell fairly rapidly, the geometric mean titre being 189 at 2-4 weeks after the rash, 27 from 3-6 months, and 8 from 12-18 months. The rapid rise and decline in CF antibody after herpes zoster infection compared with the unchanging CF titres associated with recurrent herpes simplex infection suggest that varicella-zoster and herpes simplex virus differ in their mechanism of latency.