Because irinotecan (CPT-11, Camptosar) is a topoisomerase I inhibitor with a broad spectrum of antitumor clinical activity, we investigated its activity in relapsed or refractory non-Hodgkin's lymphomas (NHLs). Irinotecan at 300 mg/m2 i.v. was administered every 21 days with intensive loperamide management of diarrhea. Responders received up to six treatment cycles. Of 44 registered patients, 32 are evaluable for response. Seventeen patients had received one previous regimen, and 15 patients had received two. Disease was refractory to the regimen preceding irinotecan in 12 patients. At baseline, serum lactate dehydrogenase levels were high in 47% (14/30), and beta-2-microglobulin levels were higher than 3.0 mg/L in 29% (8/28) of patients. Responses were seen in 12 of 32 (38%) patients (95% confidence interval [CI] = 21%-56%). Response rates were 43% for seven indolent (95% CI = 10%-82%), 0% for three mantle cell (95% CI = 0%-71%), 44% for 18 relapsed aggressive (95% CI = 22%-69%), and 20% for five refractory aggressive NHLs (95% CI = 1%-72%). Grade 3/4 toxicities included myelosuppression, neutropenic fever, and diarrhea. Irinotecan appears active and relatively well tolerated in patients with relapsed aggressive or indolent NHL. Accrual to this study is continuing for better determination of response rates in all histologic subtypes of NHL.