The present prospective multicentre trial investigated whether topotecan, given at a starting dose of 1.25 mg.m(-2) with individual dose adjustment, can improve safety in patients with relapsed/refractory small cell lung cancer without loss of efficacy. Patients received topotecan intravenously on days 1-5, every 21 days, for up to six courses. In the absence of relevant haematotoxicities, topotecan was increased to 1.5 mg.m(-2) and reduced to 1.0 mg.m(-2) in case of severe haematotoxicities. Of 170 recruited patients, 73.2% had stage IV disease and 63.4% had platinum-containing pre-treatment. Patients received a total of 521 courses. In 72.6% of those courses, the dose remained at 1.25 mg.m(-2); in 9.1% it was reduced and in 18.3% it increased. Overall response rate was 14.1% including one complete response; 28.8% had stable disease. Median duration of response was 13.6 weeks and median survival was 23.4 weeks. Clinical benefit was obvious for sensitive as well as for refractory patients. Haematotoxicity of grade 3 or 4 was clearly lower compared with the standard dose of 1.5 mg.m(-2). In conclusion, topotecan at a dose of 1.25 mg.m(-2) appears to be as effective as the dose of 1.5 mg.m(-2), but with reduced toxicity. Since patients with recurrent small cell lung cancer have a poor prognosis, they benefit especially from good tolerability.