This phase-II trial will investigate the efficacy, safety and the tolerability of a sequential therapy consisting of 4 courses of single agent rituximab followed by 4 courses of R-CHOP chemotherapy in patients with CD20+ posttransplant lymphoproliferative disorders (PTLD). However, responders to rituximab achieving a CR after the first 4 applications of rituximab will go on with rituximab monotherapy and will not receive chemotherapy.