Fifty-six of 67 patients with antibody to cytomegalovirus before transplantation shed cytomegalovirus from urine and/or saliva postoperatively. Symptomatic reactivation occurred in 17 (25%) patients, five of whom had pneumonitis. The symptomatic patients were more likely to have received a cadaver kidney (P = 0.004) and high-dosage antithymocyte globulin (P = 0.003) and to be viremic (P < 0.0001), compared to patients with silent infection. Forty-eight of 49 patients treated with antithymocyte globulin received cadaver or parent donor kidneys. Twenty-four were given a low-dosage intramuscular regimen, and 25 received a higher dosage intravenously. In the latter group 48% experienced symptomatic reactivation and 48% viremia, compared to 21% and 17%, respectively, in the former group (P < 0.05 for both comparisons). There were no symptomatic cytomegaloviral infections among 18 patients not treated with antithymocyte globulin, all of whom received related donor kidneys. Renal transplant patients who receive both a poorly matched graft and antithymocyte globulin are at increased risk of morbidity due to cytomegalovirus.