An early phase II study of topotecan produced favorable results in a small number of untreated and previously treated patients with small-cell lung cancer (SCLC). This multicenter study was conducted in patients with relapsed SCLC at 19 medical institutions in Japan. Topotecan 1.0 mg/m2/day was administered for 5 consecutive days every 3 weeks. Fifty-three patients were enrolled in the study. One patient was withdrawn before the commencement of study treatment, and 2 patients were unable to continue study treatment due to an interruption in the supply of study medication. The response rate was 26.0% in 13 of the 50 evaluable patients who were eligible and completed protocol-specified treatment and procedures. The median time to progression and overall survival were 133 days and 262 days, respectively. The most frequently reported toxicity was reversible myelosuppression, such as leukopenia, neutropenia, anemia (decreased hemoglobin), and thrombocytopenia. Nonhematological toxicity was also reported but the incidence of grade 3/4 symptoms was low. The results of this study indicate that topotecan is effective against relapsed SCLC with good tolerability.