BACKGROUND The role and timing of radiotherapy for optimal treatment of localized aggressive non-Hodgkin's lymphoma (NHL) is controversial. We report the long-term results of a single-institution pilot study of alternating chemotherapy (CT) and radiotherapy (RT) in patients with clinical stages I or II tumors exceeding 5 cm. METHODS From 1981 to 1992, 96 patients with stages I-II aggressive NHL received an alternating regimen of CT and RT consisting of 8 cycles of CT with 3 courses of RT interjected after the 2nd, 3rd and 4th cycles of CT. The CT combined cyclophosphamide, doxorubicin, teniposide and prednisone every 28 days. Each RT course was started 8 to 10 days after CT (15 Gy in 6 fractions to initially involved and contiguous areas). RESULTS The median age was 54 years. The disease predominantly located in the head and neck area was stage II in 63% of patients. Bulky tumors (10 cm or larger) were found in 24% of patients. Six patients discontinued CT because of acute toxicity (mucositis). The mean relative dose intensity achieved for doxorubicin, cyclophosphamide and teniposide were 72%, 82%, and 78%, respectively. Late toxicity consisted mostly of severe xerostomia lasting more than 2 years in 7 patients irradiated in Waldeyer's ring. The complete response (CR) rate was 91%; 20 of the 86 patients in CR relapsed (3 locally only). The median follow-up was 61 months, and at 5 years, overall survival (OS) was 77%. Classification according to the International Prognostic Factor Index was possible for 54 patients, all but three of whom were in the 'low risk' group (0-1 factor). Bulky disease was the only unfavorable prognostic factor (P < 0.001) for CR, freedom from progression (FFP) and OS rates; the low relative dose intensity of CT achieved in this study did not affect outcome. CONCLUSIONS Alternating chemo-radiotherapy for localized aggressive NHL was feasible and yielded long-term results comparable to those obtained with standard treatments, despite a reduction in dose intensity considerably below that of CHOP which suggested synergistic effects of CT and RT in this scheme.