A 12-year-old Japanese boy who underwent kidney transplantation with a kidney from his mother developed severe proteinuria immediately after the operation. Because his original disease was nephrotic syndrome (focal segmental glomerulosclerosis, or FSGS) and electron microscopic examination of the renal biopsy showed foot process fusion, we diagnosed this as a recurrence of nephrotic syndrome to the transplanted kidney. Four months after the transplantation, posttransplant lymphoproliferative disorder (PTLD) developed, which was pathologically diagnosed as diffuse large B cell lymphoma. Treatment consisting of a reduction in immunosuppression resulted in improvement in PTLD a month after the start of treatment. However, relapse occurred 2 months after the first onset of PTLD, which we treated with rituximab (CD-20 monoclonal antibody 375 mg/m2) once weekly for a total of four doses. The PTLD resolved immediately after the rituximab treatment was started, and, interestingly, urinary protein levels also improved at the same time. Three years later, the boy shows no signs of PTLD, and no proteinuria has been detected. These findings suggest that rituximab may be an effective treatment for recurrence of nephrotic syndrome after transplantation and that activated B cells may play a pivotal role in the recurrence of nephrosis after renal transplantation.