BACKGROUND Although herpes simplex virus (HSV) has been detected in the peripheral blood of immunocompromised patients and in neonates with disseminated disease, the extent to which this virus may be present in the blood during a localized infection in otherwise healthy adults is unknown. OBJECTIVE The purpose of this study was to determine whether HSV may be detected in the peripheral blood during acute recurrent herpes labialis. METHODS Peripheral blood mononuclear cells (PBMCs) were obtained from otherwise healthy adults with recurrent herpes labialis, both during an acute episode and several weeks after the lesions had healed. The PBMCs were examined for the presence of HSV with the polymerase chain reaction (PCR) and viral culture. RESULTS By PCR, HSV DNA was detected in 7 of 34 specimens from an acute episode but in none of 24 specimens in the convalescent stage (p less than 0.004). PBMCs from seven donors, who were seronegative for HSV, were also negative for HSV by PCR. Viral cultures of 22 PBMC specimens were negative (including four specimens that were positive by PCR). CONCLUSION The presence of HSV DNA in the blood is a transient phenomenon limited to the period of active infection in a minority of patients with herpes labialis, although it may be important in the development of disseminated disease as well as in the pathogenesis of herpes-associated cutaneous processes such as erythema multiforme.