Background: Reduced doses of cytotoxic chemotherapy or standard-dose therapy plus a myeloid colony-stimulating factor decreases hematologic toxicity and its complications in patients with non-Hodgkin's lymphoma associated with infection with the human immunodeficiency virus (HIV). However, the effect of reducing the doses of cytotoxic chemotherapeutic agents on clinical outcome is not known. Methods: We randomly assigned 198 HIV-seropositive patients with previously untreated, aggressive non-Hodgkin's lymphoma to receive standard-dose therapy with methotrexate, bleomycin, doxorubicin, cyclophosphamide, vincristine, and dexamethasone (m-BACOD) along with granulocyte-macrophage colony-stimulating factor (GM-CSF; n=94) or reduced-dose m-BACOD with GM-CSF administered only as indicated (n = 98). Result: A complete response was achieved in 39 of the 94 assessable patients assigned to low-dose therapy (41 percent) and in 42 of the 81 assessable patients assigned to standard-dose therapy (52 percent, P=0.56). There were no significant differences in overall or disease-free survival; median survival times were 35 weeks for patients receiving low-dose therapy and 31 weeks for these receiving standard-dose therapy (risk ratio for death in the standard-dose group, 1.17; 95 percent confidence interval, 0.84 to 1.63; P=0.25). Toxic affects of chemotherapy rated grade 3 or higher occurred in 66 of 94 patients assigned to standard-dose therapy (70 percent) and 50 of 98 patients assigned to low-dose treatment (51 percent, P=0.008). Hematologic toxicity accounted for the difference. Conclusions: As compared with treatment with standard doses of cytotoxic chemotherapy (m-BACOD), reduced doses caused significantly fewer hematologic toxic effects yet had similar efficacy in patients with HIV-related lymphoma. Dose-modified chemotherapy should be considered for most HIV-infected patients with lymphoma.