Retrospective analysis: This open controlled non-randomized study was carried out to investigate the influence of intravenous acyclovir (ACV) on the incidence of post-herpetic neuralgia (PHN). Twelve women and 11 men (mean age 52 years, range 19-89) received ACV 5 mg/kg every 8 hours) for 10 days (I). Twenty-seven untreated patients (mean age 62 years, range 20-89) were taken as a control group (II). Six to 24 months after the onset of herpes zoster (shingles) the patients were reexamined. The analysis revealed a significantly lower incidence of both general pain and severe pain (P < 0.05, chi2 = 5.55 and 4.39) for (I) compared to (II). For 21 patients who were treated for a period of 10 days, the significance level was 1% (chi2 = 7.82 and 8.62). Observational study: Fifteen thousand eight hundred and thirty-one non-hospitalized patients with shingles (mean age 55.2 years) received oral ACV (800 mg five times daily) for 7 days. At the onset of therapy, 15,420 patients (97.6%) reported pain (severe 42.6%, moderate 43.1%, mild 14.3%). The pain during treatment was documented by the patients (n = 5,728) in a diary and transferred to a scoring system (0 = none, 1 = mild, 2 = moderate, 3 = severe). From day 1 to day 7 there was a decrease in the pain score level from 2.3 to 0.9. Three months after the onset of herpes zoster, 2,519 of 14,858 patients (16.95%) reported pain; 311 patients (2.1%) complained of continuous pain, typical for PHN. The conclusion is that the incidence of PHN can be reduced if ACV is given for 7-1 0 days. (C) 1993 Wiley-Liss, Inc.