Seventeen cases of resectable peripheral small cell undifferentiated carcinoma of the lung were studied (average size, 2.1 cm; range, 0.9-3.5 cm). The carcinomas exhibited predominantly mixed intermediate and fusiform/spindle subtypes; in seven carcinomas there were at least some foci of oat cell subtype. There was no histologic evidence of typical carcinoid tumor. Seven cases were fixed in solutions appropriate for immunohistologic study; these exhibited prominent neuron-specific enolase activity but less prominent and more variable staining for the carcinoembryonic antigen and cytokeratins. All 17 cases were deemed resectable and had no clinical or radiologic evidence of metastasis. Seven (41%) patients died with recurrent and/or metastatic carcinoma (average survival, 1.7 years); five of these patients had carcinomas with at least some foci of oat cell foci subtype.