The incidence of Epstein-Barr virus (EBV) infection and lymphoproliferative disorder (LPD) was determined in a pediatric liver transplant population consisting of 51 children treated with FK506 and 91 treated with cyclosporine. The incidence of symptomatic EBV infection was 21.9% (23 of 105 cases) in children < 5 yr old and 10.8% (4 of 37 cases) in children 5 to 17 yr old as compared with 2.7% (9 of 323 cases) in adults (P < 0.0001). In the under 5 yr old group on cyclosporine, the incidences of EBV infection and LPD were 9 of 68 (13.2%) and 2 of 68 children, (2.9%), respectively. In contrast, in children under 5 yr old group on FK506, the incidences of EBV infection and LPD in the FK506 group were 14 of 37 (37.8%) and 7 of 37 children (18.9%), respectively. The difference between these two groups was statistically significant (P < 0.02). There were no cases of LPD in the 5-17 yr-old children on either cyclosporine (n = 23) or FK506 (n = 14). The incidence of EBV infections in the 5 to 17 yr age group, 17.4% on cyclosporine and 0% on FK506, was less than for the younger children on FK506 (37.8%). A total of 39% (9 of 23) of children under 5 yr old who had symptomatic EBV infections developed LPD, and 44% (4 of 9) with LPD died. The higher incidence of EBV infections and LPD in the younger children treated with FK506 was probably related to a greater intensity of immunosuppression for patients on FK506 than those on cyclosporine.