2-Chlorodeoxyadenosine (Cladribine) is a new purine analogue with high activity in pretreated low grade non-Hodgkin's lymphoma (NHL). To evaluate the efficacy of this drug in untreated patients with advanced NHL, we performed a prospective multicentre trial. Cladribine (0.12 mg/kg) was administered intravenously daily for 5 consecutive days in an out-patient setting. The treatment was repeated every 28 days for four cycles. Included were patients with a histological diagnosis of low grade NHL according to the Kiel classification and stage III or IV disease. Stage II patients were included when radiotherapy had failed. 55 patients were entered into the study. 50 patients were evaluable. The remission rate was 44/50 (88%; 95% confidence interval 82-100%), including complete remissions (CR) in 14 (28%) patients. Only 2 patients showed progression while on Cladribine treatment. The estimated overall survival, and time to treatment failure (TTF) were 85% and 51%, respectively, after a median observation time of 92 weeks. 11 (22%) patients showed grade 3 or 4 toxicity according to the WHO grading. Haematological toxicity was responsible for 86% of the overall toxicity and 100% of grade 3 and 4 toxicity. 7 patients (14%) had an infection, two of which were opportunistic. 12 (24%) patients did not experience any toxicity during the treatment. The results of this study clearly demonstrate the safety and considerable activity of this regimen. Cladribine is very effective even at lower doses than have been used so far.