Fifty-nine orthotopic liver transplant (OLT) patients were studied after transplantation to detect Epstein-Barr virus (EBV) primoinfection and reactivation. Nineteen, all children under 10 years, were EBV seronegative. Seroconversion occurred in 12 (63.3%) of the seronegative patients. Most of these patients (10/12) seroconverted 2 or 3 months after transplantation; 11 out of the 12 demonstrated clinical signs at the time of seroconversion. From 9 primoinfected patients tested for EBV excretion, 8 were found to be positive. Serological evidence of reactivation was found in 9 out 40 (22.5%) seropositive patients and EBV was isolated from 5 (56%). Eleven pediatric OLT patients with primoinfection showed high and persistent titers of anti-EA antibodies (from 1:32 to greater than or equal to 1:256), when tested at least 3 months after seroconversion; however, anti-EBNA antibodies failed to develop in 5 patients and remained persistently low in 4. These patients with high EA and with negative or low EBNA titers constitute an "at risk" group for EBV-related lymphoproliferative syndrome (LpS). At presently, after a period of follow-up ranging from 3 months to 3 years, none of our 12 primoinfected patients have developed any lymphoproliferative evolution. However, in 1, during the acute phase, lymphoblasts and lymphoproliferation were observed in a tonsil biopsy.