The rationale for performing the present study is to combine two highly active treatment modalities in first line therapy of solid organ recipients with B-cell PTLD. The monoclonal antibody CD20 represents an effective therapeutic approach in the treatment of PTLD. Unfortunately this effect seems to be of limited duration in some patients, who benefited from monotherapy with rituximab. The advantage of this therapeutic approach in PTLD is due to the low incidence of third to fourth degree adverse events. At diagnosis of PTLD a relevant proportion of these patients is not suitable for first line cytotoxic chemotherapy due to widespread disease, organ dysfunction or reduced performance state. Insufficiencies of kidney or bone marrow function are frequent in organ recipients due the toxic side effects of the immunosuppressive drugs. After pre-phase treatment with the monoclonal antibody rituximab the CHOP chemotherapy is suggested to be less toxic due to the lower tumor burden. Thereby treatment related severe or even lethal toxicities, frequently reported in patients with PTLD who underwent cytotoxic chemotherapy, may be prevented. Furthermore the total number of cytotoxic cycles of CHOP-therapy is reduced from 6 for 8 to 4 cycles and thus may result in an additional reduction of toxicity in the single patient. Because immunochemotherapy (R-CHOP) is clearly superior to CHOP with respect to progression free survival and relapse rates in patients with classical NHL and rituximab is very unlikely to add any further toxicity to CHOP the majority of patients will go on with four courses of R-CHOP. However, patients with a complete remission after 4 courses of single agent rituximab may have a very favourable risk profile and therefore will go on with rituximab single agent instead of R-CHOP.