In order to assess the accuracy of bronchial aspiration cytology in typing lung cancer, tissue sections from 100 autopsy cases of lung cancer were compared with the cytology features observed in the same patients prior to death. There was 100% accuracy in the cytology of small-cell carcinoma; 90% in squamous-cell carcinoma; 70% in adenocarcinoma; and 50% in undifferentiated large-cell carcinoma. The observed discrepancies probably reflect intrinsic tumor properties rather than problems attributable to either the bronchial aspiration method or cytology interpretation, especially in cases involving advanced lung carcinoma. Because the highest accuracy rate was in detecting small-cell carcinoma, it is recommended that only the distinction between small-cell and non-small-cell forms be made on cytologic grounds and that further categorizations only be rendered in cases with unquestionable cytomorphological features.