This prospective study examines 42 children in the first year after renal transplantation. They all received intravenous ganciclovir prophylaxis for cytomegalovirus in the immediate post-transplant period. Quantitative antigenemia (pp68) determinations and blood, urine and throat cultures were done on a scheduled basis to detect cytomegalovirus. Infection was detected in 22 children (52.4%) within an average 44.31 +/- 27.38 days; 5/22 were symptomatic. The antigenemia was positive (+) in all the infected patients, and so were blood culture in 68.2%, urine culture in 59.1% and throat culture in 31.8%. A positive antigenemia was the earliest finding in all cases but 1. The 5 children with clinical symptoms received intravenous ganciclovir. Asymptomatic infected children received oral ganciclovir at an average dose of 47.64 +/- 8.10 mg/kg/day (median 46.58 (range 33-58.7) mg/kg/day) for an average of 58.47 +/- 27.76 days (median 58 (range 26-211) days). No patient developed disease or ganciclovir resistance during the treatment. No patient presented acute graft rejection or renal dysfunction and their glomerular filtrate rate at 1 year was similar to that of noninfected children (90.38 +/- 26.51 vs. 93.93 +/- 36.24 ml/min/1.73 m2). We conclude that preemptive treatment with oral ganciclovir is useful and safe in children with renal transplantation and that monitoring blood antigenemia is a sensitive and early method to detect and control CMV infection.