TCNU, a new water soluble nitrosourea, has in preclinical studies shown higher activity when given as divided doses compared with one day single treatment. Accordingly, 38 consecutive patients with non-resectable adenocarcinoma of the lung received TCNU 40 mg/m2 p.o. daily for three days every 4 weeks. The response rate among 37 evaluable patients was 14%, median response duration was 17 weeks (range 5-57+ weeks) and median survival 22 weeks (range 3-96+ weeks). Hematologic toxicity was pronounced, especially thrombocytopenia, and 57% of the patients had WHO grade 3 or 4 WBC or platelet count. Overall, dose reduction or delay of treatment due to hematologic toxicity was necessary in 66% of the patients. This, together with the limited activity, renders TCNU in the present dose and schedule unsuitable for further investigations in this disease entity.