Forty-eight patients were recruited into a study of continuous oral acyclovir therapy for the suppression of recurrent genital herpes simplex virus (HSV) infection. Seven of these patients were shown to shed HSV in the absence of clinical signs or symptoms whilst on medication. The asymptomatic shedders did not differ significantly from the rest of the group in terms of age, interval from first attack to enrollment or number of recurrences prior to enrollment. Only one patient admitted to poor compliance as a trigger to asymptomatic HSV shedding. Failure to suppress asymptomatic shedding during prophylactic acyclovir therapy may have implications for sexual transmission to partners and vertical transmission to neonates and requires further study.