Forty-nine recipients of renal allografts were studied for infection with herpes simplex virus (HSV) before and at sequential intervals after transplantation. Forty-four (90%) of the patients studied were initially seropositive for neutralizing antibody to HSV type 1. HSV was not shed prior to transplantation nor by any of the five seronegative recipients after transplantation. Twenty-nine (66%) of the 44 seropositive patients shed virus postoperatively: 23 in saliva, three in urine, and three in both sites. Twenty (63%) of 32 seropositive patients examined developed herpetic mucocutaneous lesions. Both viral shedding and lesions were most prevalent during the first four weeks after transplantation. Twenty-nine (85%) of 34 patients with antibody titers of 1:256-1:4, 096 and zero of 10 with titers of 1:8-1:128 shed HSV postoperatively (P < 0.0001). The group with high antibody titers before transplantation were also more likely to develop lesions after transplantation (P = 0.002) as were those with a positive history (P = 0.017). The ability to predict symptomatic HSV recurrences in renal transplant patients could be a valuable aid in identifying individuals with which to evaluate antiviral compounds. © 1979 by The University of Chicago.