MCs are important effector cells in a broad range of immune responses. Their role in liver allograft rejection is not clear. Twenty-one liver transplant recipients (mean age +/- s.d.; 10.2 +/- 4.1 yr) who experienced a rejection episode are included in this study. Biopsy specimens from normal livers (allograft biopsy with normal histopathology n = 5 and naive livers n = 6), transplanted livers with CR (n = 5), and transplanted livers with ACR (n = 26) were studied. The total number of PT in each biopsy specimen was documented, and the number of PT that contained MCs was expressed as a percentage of the total number of PT. MCs, percentage of PT containing MCs and the average number of MCs/PT was significantly higher in rejection specimens than in control biopsy samples. All parameters were significantly higher in CR group than AR groups. Increasing grades of rejection was also associated with progressively more MCs and MC/PT (r = 0.68 p = 0.000; r = 0.58 p = 0.002). Only serum bilirubin level was related to the MCs in AR group. Only MC/PT was detected as an independent predictor of graft survival (p = 0.011, RR 2.87 95% CI 1.3-6.5). Despite the fact that the role of MCs in liver allograft rejection is still unknown; they exist in inflammatory infiltrates during pediatric liver allograft rejection. MC-rich portal infiltrates may distinguish chronic liver rejection from other inflammatory states such as AR, hepatitis and biliary obstruction.