30 patients with small cell lung cancer (SCLC) and malignant pleural effusion were compared with 30 matched patients with SCLC but without pleural effusion. In the 30 with pleural effusion, white blood cell and platelet counts fell significantly after initial chemotherapy, necessitating dose reduction. Of the patients with pleural effusion, 16 developed severe (WHO grade IV) leukopenia, 7 had severe thrombocytopenia, and 2 patients died of infection. Accordingly, exhaustive aspiration of radiologically verified pleural effusion before starting chemotherapy in patients with SCLC is recommended.