One hundred thirty specimens from 108 consecutive patients with a history of recurrent genital ulcerations were used in a study comparing herpes simplex virus (HSV) isolation with a direct fluorescent-antibody (DFA) technique using mouse monoclonal antibodies. HSV was isolated from 70% of vesicular lesions, 67% of pustular lesions, 32% of ulcerative lesions, and 17% of crusted lesions, whereas the DFA technique detected HSV antigen in 87, 67, 30, and 10% of lesions in similar stages, respectively. When both methods were used, HSV was identified in 97, 79, 45, and 17% of vesicles, pustules, ulcers, and crusted lesions, respectively. The overall sensitivity and specificity of the DFA technique in comparison with virus isolation (VI) were 74 and 85%, respectively. Of the 17 patients from whom DFA-positive, VI-negative samples were obtained, HSV was subsequently isolated from a genital lesion in 14, suggesting that they were not DFA false-positives. Similarly, of the 46 patients whose initial lesion samples were DFA and VI negative, 37 (80%) had HSV identified from subsequent genital lesions during follow-up. Thus, a single sample for VI or DFA testing from a recurrent genital lesion had a sensitivity of only 53 and 51%, respectively. Combining the DFA technique and VI increased the sensitivity of laboratory diagnosis of a single recurrent episode of genital HSV; however, repetitive laboratory testing was often required to confirm the diagnosis of recurrent genital HSV infection.