We conducted a phase II study to evaluate the efficacy and safety of cladribine (2-chlorodeoxyadenosine [2-CdA]) for patients with refractory or relapsed indolent B-cell lymphoma or mycosis fungoides. Forty-five patients were enrolled, and 43 patients, including 34 with follicular lymphoma, were eligible. 2-CdA was given by continuous intravenous infusion at a dose of 0.09 mg/kg daily for 7 consecutive days, and this schedule was repeated every 4 weeks up to a maximum of 6 cycles. The overall and complete response rates were 58.1% (25/43; 90% confidence interval, 44.5%-70.9%) and 14.0% (6/43), respectively. The disease progression-free proportions of all 43 eligible and all 25 responding patients at 2 years were 30.3% and 48.1%, respectively. Neutropenia and thrombocytopenia of grade 3 or 4 were observed in 53.3% and 37.8% of patients, respectively, with prolonged cytopenia observed in patients with increased numbers of treatment cycles. Nonhematologic toxicities of grade 3 or greater included diarrhea, arrhythmia, malaise, and gastrointestinal bleeding in 1 patient each, an increase in glutamic-pyruvic transaminase level in 2 patients, and infection in 5 patients. Two treatment-related deaths were observed. Four patients developed myelodysplastic syndrome (MDS) at 13 months to 2 years after completion of the 2-CdA treatments. 2-CdA is an active agent with acceptable toxicity for refractory or relapsed indolent lymphoma; however, prolonged myelosuppression and the potential development of MDS should be carefully monitored.