INTRODUCTION: Denileukin diftitox, a chimeric protein, uses the cytocidal properties of diphtheria toxin to cells expressing interleukin-2 receptors. The aim of this study was to evaluate the efficacy and safety of denileukin diftitox in the treatment of advanced relapsed nonsmall cell lung cancer (NSCLC). PATIENTS AND METHODS: Multicenter phase II trial in patients with NSCLC with Eastern Cooperative Oncology Group PS 0-2, stage IIIB/IV at diagnosis, who had failed at least 1 previous chemotherapy regimen. Denileukin diftitox was infused at 18 microg/kg/d x 5 days, every 21 days for 6 cycles. RESULTS: For the 41 patients enrolled, the median age was 56 years (range, 21-80), 25 were men, and the median number of previous chemotherapy regimens was 2 (range, 1-5). The median number of treatment cycles was 2 (range, 1-6). By RECIST criteria, 18 (44%) had stable disease, 10 (24%) progressive disease, and 13 (32%) were not evaluable for response as they received less than 2 treatment cycles. The median time to disease progression was 1.8 months [range, 0.3-11.3; 95% confidence interval (CI) 1.3-2.6]. Median survival was 5.8 months (range, 0.3-33.6; 95% CI 3.4-11.4). The median follow-up time was 16.1 month. One death from myocarditis verified at autopsy was attributed to treatment. One grade 4 toxicity (vascular leak syndrome) was encountered, and 18 grade 3 toxicities, primarily gastro-intestinal, vascular leak syndrome, and constitutional symptoms. CONCLUSION: Denileukin diftitox at current dose schedule has limited activity in patients with previously treated NSCLC, manifested by disease control without impact on survival.