This multicenter, randomized, open-label, parallel-group study will evaluate the efficacy and safety of subcutaneously administered rituximab in comparison with observation only as maintenance therapy in participants with relapsed or refractory indolent Non-Hodgkin's lymphoma (NHL). All participants will receive induction therapy with rituximab (375 milligrams per square meter [mg/m^2] intravenously [IV] in Cycle 1, then 1400 mg subcutaneous [SC] every 3-4 weeks) plus standard chemotherapy for 6-8 months; followed by 24 months of maintenance I period with rituximab (1400 mg SC every 8 weeks). Participants completing therapy and showing partial or complete response will be randomized to receive either rituximab (1400 mg SC every 8 weeks) or observation with no treatment during maintenance II period and will be followed for at least 15 months. Anticipated time on study treatment is until disease progression, unacceptable toxicity or end of study, whichever occurs first.