Thirty-two patients with refractory non-Hodgkin's lymphoma were treated with high-dose cytosine arabinoside (ara-C) given at 2 g/m2 IV over three hours every 12 hours for 4-8 g/m2/course repeated at three to four week intervals. There were eight partial responses (29%) and two minor responses among 28 evaluable patients. The median response duration was 10 weeks (range, 6-33 weeks). The median survival was significantly prolonged in responders compared to nonresponders (28 versus 15 weeks; p = 0.03). Two additional patients treated with 12 g/m2/course died of sepsis and myelosuppression. The dose-limiting toxicity was myelosuppression, which was more pronounced in patients with prior extensive radiation therapy and bone marrow involvement. In vivo measurements of intracellular concentrations of ara-CTP, the active metabolite of ara-C, showed significantly higher values in bone marrows with lymphomatous involvement compared to normal bone marrows (210 versus 95 microM; p = 0.05), probably indicating a preferential formation and retention of ara-CTP in malignant cells compared to normal hemopoietic cells. In addition, higher ara-CTP levels were found in bone marrows that had higher percentages of cells in S phase.