To define risk factors associated with recurrent genital herpes-simplex-virus infection caused by either Type 1 or 2 herpesvirus (HSV-1 or HSV-2), we prospectively studied 137 patients with a first symptomatic episode of the disease and 87 with a recurrent episode. First episodes were divided into 78 primary infections (no antibodies to HSV in acute-phase serum) and 59 nonprimary infections (antibodies present). HSV-1 infections were less frequent and less likely to recur than HSV-2 infections. Fifteen per cent of primary first episodes were caused by HSV-1, as compared with 3 per cent of nonprimary first episodes and 2 per cent of recurrent episodes. Moreover, during follow-up of first-episode patients, only 14 per cent of HSV-1 infections recurred, as compared with 60 per cent of HSV-2 infections. Recurrences were more likely to follow an index recurrent episode than an index first episode, whether primary or nonprimary, and were more likely to occur in men than in women. Among patients with primary HSV-2 infections, the probability of recurrence was directly related to the presence and titer of neutralizing antibody to HSV-2 in convalescent-phase serum.