One hundred ninety-three patients with untreated advanced small cell anaplastic carcinoma of the lung were divided into two groups on the basis of whether or not the pretreatment bone marrow examination revealed tumor. The groups were analyzed with regard to survival, treatment response, tolerated amount of chemotherapy, hematologic toxicity including necessity for blood transfusions, and episodes of clinical sepsis. The median survival time was significantly lower for patients with a positive bone marrow examination (149 days vs. 231 days) (P less than 0.01). Short survival times were especially observed for patients with bone marrow metastases and initial thrombocytopenia as compared to those with bone marrow metastases but normal platelet counts (68 days vs. 176 days) (P less than 0.05). The median duration of remission was 86 days for the groups with positive bone marrow examinations vs 182 days for the negative group (P less than 0.05). No difference was observed with regard to other parameters studied. It is concluded that a positive pretreatment bone marrow examination is an unfavorable prognostic indicator, especially for patients with additional thrombocytopenia, but it does not seem to be a restrictive factor for effective use of intensive chemotherapy in the treatment of small cell anaplastic carcinoma of the lung.