This study explored the efficacy and toxicity of the combination of pentostatin and rituximab, effective single agents in low-grade non-Hodgkin's lymphoma (NHL). Sixty patients with previously treated low-grade NHL were enrolled. Except for day 1, both drugs were administered weekly for 4 weeks, with week 5 off. During week 1 (day 1) only rituximab was given; subsequent weekly treatments included both drugs. Patients received a minimum of 2 five-week cycles in order to be evaluable for efficacy. Responses were evaluated on week 5 of cycle 2. If partial response (PR) or stable disease (SD) responses were noted, 2 additional cycles were administered. Final evaluations were done on week 5 of cycle 4. Of 60 patients, 58.3% had an Eastern Cooperative Oncology Group performance status (PS) of 0, and 41.7% had PS of 1; 31.7% and 51.7% had stage III or stage IV disease, respectively. Histology included follicular center, follicular, grade I (45%), II (21.7%), III (1.7%), and small lymphocytic (31.7%). Seventeen patients had prior chemotherapy, but no patients had received prior pentostatin or rituximab. Median age was 60.3 years (range, 32.5-84.7 years). Among 57 evaluable patients, 77% responded (22.3% complete response [CR], 3.5% unconfirmed CR, 35.1% PR, and 10.5% unconfirmed PR); 19.3% had SD, and 8.8% progressive disease (PD). Response rate among previously untreated patients was 83% versus 63% in previously treated patients. Median duration of response was 11 months (range, 2.3-22.2 months); median time to progression was 15 months (range, < 1-25 months). Neutropenia was the only adverse event experienced by >/= 10% of patients. Six deaths were caused by PD, and one death each was caused by acute respiratory distress, possibly related respiratory failure, and cardiac toxicity. These results suggest the combination of pentostatin/rituximab is well tolerated and active in low-grade lymphoma.