Forty-four patients, with low-grade non-Hodgkin's lymphoma (LG-NHL) were included in a phase II study between June 1993 and May 1995 and treated with cladribine (CdA) 0.12 mg/kg as a 2 h i.v. infusion daily x 5, repeated after 28 days for up to 6 courses. Thirty-four patients were previously untreated and 10 had progressive disease after initial response to limited chlorambucil treatment. Five patients had also received involved field radiotherapy. Eight patients had mantle cell lymphomas, 22 follicle centre lymphomas, 5 lymphoplasmacytoid lymphomas, 4 small cell lymphocytic lymphomas, 4 marginal zone B-cell lymphomas and I had unclassified low-grade NHL. The response rate was 64%, with 11 (25%) CR and 17 (39%) PR while 5 (11%) patients progressed during treatment. The response rate was similar in previously treated and untreated patients. The median number of CdA courses delivered was 3 (1-6) in non-responding patients and 6 (2-6) in responders. Median survival from inclusion was not reached with a median follow-up of 40 months. The median time to progression was 7 mo for all patients, 25+ mo for CR and 16 mo for PR patients. Toxicity was sometimes severe with 2 treatment related deaths, one infectious related and one due to a mucocutaneous syndrome and pulmonary microembolism. In addition, 5 grade 3 or 4 infectious episodes were seen. Seven patients experienced grade 3 or 4 thrombocytopenia and 20 had grade 3 or 4 neutropenia. We conclude that the majority of patients with low-grade non-Hodgkin's lymphoma respond to CdA but that the adverse effects may be severe.