Pulmonary toxicity may complicate the treatment of non-Hodgkin's lymphoma (NHL). The possible drug-related cause of pulmonary toxicity was investigated retrospectively in 207 NHL patients treated between 1981 and 1988 with three regimens containing cyclophosphamide with and without methotrexate or bleomycin: methotrexate, calcium, leucovorin, bleomycin, doxorubicin, cyclophosphamide, vincristine, and dexamethasone (m-BACOD) (n = 134); methotrexate, calcium, leucovorin, doxorubicin, cyclophosphamide, vincristine, and dexamethasone (m-ACOD) (n = 43); or cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) (n = 30) chemotherapy. These regimens contained the same drugs and were administered in the same schedule; the regimens differed primarily in the addition of bleomycin or methotrexate. Pulmonary toxicity occurred in 24 of 134 (18%) m-BACOD-treated and in six of 43 (14%) m-ACOD-treated patients (P = 0.65). Chest radiography revealed diffuse pulmonary infiltrates in 16 (67%) and six (100%) of the m-BACOD-treated and m-ACOD-treated patients with pulmonary toxicity, respectively. None of the CHOP-treated patients had pulmonary toxicity. The clinical features of pulmonary toxicity and the amount of chemotherapy administered before it occurred did not differ in patients treated with m-BACOD or m-ACOD, although the toxicity tended to be more severe in the m-BACOD group. Open lung or transbronchial biopsies done in six (38%) of the m-BACOD-treated and three (50%) of the m-ACOD-treated patients with pulmonary infiltrates revealed nonspecific pneumonitis compatible with drug-related toxicity. In summary, these results showed that pulmonary toxicity during m-BACOD and m-ACOD therapy occurred with similar frequency and clinicopathologic features. This suggested that bleomycin was not responsible uniquely for the pulmonary toxicity in m-BACOD-treated patients. That pulmonary toxicity was not observed in patients treated with CHOP suggested that methotrexate may play an important role in the pathogenesis of the pulmonary toxicity.