A consecutive series of 104 autopsies on patients treated in protocol for small cell carcinoma of the lung (SCCL) was studied with respect to metastatic pattern at autopsy in relation to pretreatment WHO 1981 classification, and extent and significance of non-SCCL tumor tissue at autopsy. The only significant difference in the metastatic pattern at autopsy between patients with pretreatment oat cell or intermediate subtype was metastases to the brain. Thus, the frequency of brain metastases was 17/35 (49%) in patients with oat cell type compared to 2/18 (11%) in patients with intermediate type (P less than 0.05). At autopsy 13 of 98 patients (13%) had non-SCCL tumor tissue in at least one site. These patients had a significantly shorter survival (P = 0.020) compared to patients with pure SCCL at autopsy. Furthermore, none of these 13 patients had obtained complete remission. Whether these morphologic variations had been present at the pretreatment stage of the disease or were due to the chemotherapeutic treatment could not be solved in the present study. However, the results might indicate that mixed SCCL/non-SCCL histology is a negative prognostic factor in the treatment of SCCL. Prospective studies including more extensive pretreatment tissue sampling seem to be required.