We have previously reported that 3'-azido 3'-deoxythymidine (AZT) can possess a significant antineoplastic activity when combined with drugs that disrupt de novo thymidylate synthesis, such as 5-fluorouracil and methotrexate (MTX). The aim of the present study was to evaluate the efficacy and the tolerance of the combination AZT + MTX in human immunodeficiency virus (HIV)-related non-Hodgkin's lymphoma (NHL). Twenty-nine patients (22 men and 7 women), either newly diagnosed or pretreated, have been enrolled in the trial; the median age was 34 years, 45% had acquired immunodeficiency syndrome before lymphoma and 19 patients had less than 100 CD4 lymphocytes/microL. Histologic diagnoses were mainly Burkitt (27%) and diffuse large B-cell lymphoma (45%); extranodal involvement was present in 20 patients. The treatment plan included three weekly courses of MTX at 1 g/m2 (days 1, 8, and 15) plus oral AZT at 2 g/m2 (days 1, 2, and 3), 4 g/m2 (days 8, 9, and 10), and 6 g/m2 (days 15, 16, and 17), plus leucovorin rescue. From the eleventh patient on, in case of complete or partial remission, the treatment was continued with three additional courses, using AZT at the maximum dose. In 26 evaluable patients, the total (complete + partial) response rate was 77% (95% confidence interval, 58% to 89%), with complete remission (CR) in 46% of the patients (95% confidence interval, 29% to 65%). The median CR duration was 12.8 months. Grade III-IV neutropenia and anemia were observed in 52% and 31% of the courses, respectively. There was one therapy-related death due to bacteremia followed by septic shock; the only other recorded infection was a herpes vaginalis. In conclusion, we suggest that AZT + MTX is an effective and well-tolerated regimen in HIV-related NHL.