KW-2149 is a new mitomycin C (MMC) analog, forming DNA-DNA and DNA-protein crosslinking 20-fold more effectively than MMC. Because of its equal or superior in vitro and in vivo activity compared to MMC, a phase I study was initiated with an intravenous bolus injection every three weeks. This study was interrupted after dose escalation from 5 mg/m2 to 100 mg/m2 because of subacute and dose dependent pulmonary toxicity. Because of the lack of other end-organ toxicity, the moderate hematological toxicity and the observed antitumor effect, a second phase I study was initiated with a 24 hour continuous infusion. The starting dose was 50 mg/m2 and further escalation depended on observed pulmonary toxicity. Four patients were entered into this study and the received in total 17 courses. Toxicity was again mainly restricted to the lungs with one patient suffering grade 2 dyspnoe and another one grade 1 dyspnoe. Three patients had a substantial change in the carbon monoxide (CO) diffusion capacity. Pharmacokinetic data from these patients showed very low plasma levels both for KW-2149, as for both known metabolites M-16 and M-18. This study demonstrates that pulmonary toxicity continues to occur with KW-2149, in spite of the assurance of low plasma levels of both the parent compound and the known metabolites. The interesting activity of this compound has stimulated further in-depth research towards mechanisms of pulmonary toxicity and means of preventing them.