A total of 44 patients with previously untreated; non-resectable non-small-cell lung cancer (NSCLC) were treated with 4'-iodo-4'-deoxydoxorubicin (IDX), which is an analogue of doxorubicin with less cardiotoxicity. Patients received 80 mg/m2 i.v. every 3 weeks. Dose reductions were carried out for haematological toxicity. Response was assessed prior to each treatment according to WHO criteria. Among the 43 evaluable patients, 1 (2%; 95% confidence limits, 0-8%) achieved a partial response. Leucocytopenia of WHO grade 3 or 4 occurred in 64% of patients and corresponding thrombocytopenia grade 3 or 4 occurred in 30%. Of the 26 patients who were evaluated by measurements of the left ventricular ejection fraction (LVEF), 4 had a decline in LVEF of more than 15%, and 2 patients developed congestive heart failure. Myocardial biopsies were not done. In conclusion, IDX is not active in NSCLC at the applied dose and on the schedule used. Moreover, it does not seem possible to increase the dose intensity further due to the observed toxicity.