Eighty-eight patients entered into the British National Lymphoma Investigation with clinical stage I and II, grade I non-Hodgkin's lymphoma were treated initially with involved field radiotherapy alone. Eighty-one per cent presented with nodal disease. The duration of follow-up was 25-116 months, with a median of 54 months. Fifteen patients died of disease and the 5-year survival of the whole group was 83%. The complete response rate was dependent on the radiotherapy dose and was greater than 90% for doses of 3500 cGy and over. Most failures occurred at distant rather than adjacent sites, suggesting that extended field radiotherapy would not have affected the outcome. Second-line treatment induced complete remission in 66% of patients who relapsed. The prognosis was significantly worse in patients with intra-abdominal disease.