OBJECTIVE: To determine the efficacy and safety of once daily valaciclovir for the suppression of recurrent genital herpes simplex virus (HSV) infection in immunocompetent patients. METHODS: 382 otherwise healthy patients with a history of frequently recurring genital HSV infection (eight recurrences per year) were randomly allocated to receive either oral valaciclovir (500 mg once daily) or placebo (3:1 ratio) for 16 weeks or until the first genital HSV recurrence, whichever occurred first. Patients were clinically assessed at regular intervals and also if they experienced a recurrence. Safety was evaluated through adverse experience reporting and monitoring of haematology and biochemistry variables. On completion of the double blind phase, patients were eligible for follow up to a maximum of 48 weeks' treatment with open label valaciclovir (500 mg once daily) for further safety monitoring. The results from the double blind phase of the study are reported here. RESULTS: A significant difference was detected between valaciclovir and placebo in the time to first recurrence of genital HSV infection. The hazard ratio [95% confidence interval] for valaciclovir v placebo was 0.155 [0.112, 0.214], p < 0.0001. Valaciclovir prevented or delayed 85% of the recurrences that would have occurred with placebo. After 16 weeks (day 112) with treatment, 69% of patients receiving valaciclovir were recurrence free compared with only 9.5% of patients assigned to placebo. The safety profiles of valaciclovir and placebo were comparable, with adverse experiences being infrequent and generally mild. CONCLUSION: This study has demonstrated that once daily valaciclovir (500 mg), is highly effective and well tolerated for the suppression of recurrent genital HSV infection. Once daily dosing with valaciclovir provides a more convenient dosing regimen than the more frequent aciclovir regimens.