A prospective study was performed to evaluate the feasibility of full dose chemotherapy given on schedule in elderly patients with unfavourable non-Hodgkin's lymphoma, stage IE, III and IV. Using a combination regimen of six courses of cyclophosphamide, mitoxantrone, vincristine and prednisone (CNOP) given every 4 weeks, no serious toxicity was encountered in a group of 30 consecutive patients with a mean age of 70.4 years. A 60% complete response rate was observed and a total response rate of 90%. The disease-free survival of complete responders was 50% at 1 year. The overall survival was also 50% at 1 year. In 148 courses of CNOP only two serious infectious episodes were noted, i.e. one herpes zoster infection and one case of bronchopneumonia. Asymptomatic transient thrombocytopenia and granulocytopenia were commonly observed. Nadirs of white blood cells were WHO grade 1, 2, 3 and 4 in six, five, twelve and two patients respectively and nadirs of thrombocytopenia were WHO grade 0 and 1 (22 patients) or 2 (three patients). Based on low white blood cell counts, a delay of 1 week before administration of the next course of CNOP was necessary in 7% of the courses. No dose reductions were applied. Toxicity other than transient granulocytopenia was minor and consisted of alopecia and nausea. WHO grade 0-2. CNOP related toxicity was never a reason to stop treatment. It is concluded that CNOP chemotherapy without initial dose reduction in elderly patients with intermediate and high grade malignant non-Hodgkin's lymphoma is feasible and that no major toxicity is observed.