The authors have studied the ultrastructural features of 52 cases of oat cell carcinoma of the lung and have related their observations to tumor stage and patient survival. Only the type of cell junctions seems to be of prognostic importance. Tumors with intermediate junctions--and especially those with desmosomes--have a more localized stage and may be resectable to result in longer survival than expected for oat cell carcinomas without junctions. For example, in the authors' series the median survival periods for those with no identifiable junctions, intermediate junctions, or desmosomes were 6.4, 8.2, and 11.3 months, respectively. Nevertheless, this ultrastructural subclassification is not as effective as that obtained from careful clinical staging.