Two multicentre phase II trials were designed to determine if tumour responses can be achieved in progressive small-cell lung cancer (SCLC) or non-small cell lung cancer (NSCLC) patients treated with ISIS 5132, an inhibitor of C-raf kinase mRNA expression (CGP 69846A; ISIS Pharmaceuticals Inc, Carlsbad, CA), and to further characterise the safety of the compound. Between August 1998 and November 1999, 26 patients (18 NSCLC, 8 SCLC) were entered. Out of these, 23 were eligible, 22 (18 NSCLC, 4 SCLC) were treated with ISIS 5132 (2 mg/kg/day, 21 days continuous intravenous (i.v.) infusion every 4 weeks) and were evaluable for toxicity and 18 (15 NSCLC, 3 SCLC) were evaluable for efficacy. For the whole group haematological toxicity did not exceed grade 2. One patient experienced a grade 4 increased prothrombin time. Non-haematological toxicity was mild to moderate, with the observation of asthenia and nausea and vomiting. Progressive disease (PD) was diagnosed in 10 patients (8 NSCLC and 2 SCLC). 8 more patients (7 NSCLC, 1 SCLC) were considered as treatment failures. In conclusion, this study using ISIS 5132 with this dose and schedule of administration excludes a 20% response rate with 95% confidence intervals for NSCLC and cannot draw any conclusions for SCLC patients as only a few were involved in the study.