This single-arm, open-label, multi-centre study will evaluate the efficacy and safety of MabThera/Rituxan (rituximab) in patients with follicular non-Hodgkin's lymphoma with minimal residual disease after autologous haematopoietic stem cell transplantation (bone marrow or peripheral blood). Two groups of patients will be considered for treatment. Group A: Patients with evaluable minimal tumor mass, Group B: Patients with complete response and abnormal B-cell lymphoma 2 (bcl2) status. Patients will receive MabThera/Rituxan 375 mg/m2 intravenously once every week. The anticipated time on study treatment is 4 weeks.