Sixty-three bronchial tumours are described. Using the criteria of necrosis, a mitotic count of 5 or greater per 10 high-power fields, well marked nuclear pleomorphism, lymphatic and vascular invasion, either singly or in combination, 38% of cases were considered atypical or well-differentiated neuroendocrine carcinoma. If an undifferentiated growth pattern was added as a further criterion, 49% of bronchial carcinoids were atypical. Oncocytic change was common (59% of cases) but only one pure oncocytic carcinoid was present in the series. Bone was seen in 25% of cases. In some cases it arose from the bronchial cartilage but in others it was seen in the centre of the tumour. The range of histological patterns seen in carcinoids is described. Rare types of bronchial carcinoids showed a papillary pattern as well as spindle cell foci. Two cell types were noted--large cells with eosinophilic cytoplasm and cells with small hyperchromatic nuclei and little cytoplasm. These cells are apoptotic as shown ultrastructurally. The differential diagnosis of these tumours is discussed. In 35 cases a clinico-pathological correlation was possible. Bronchial carcinoids are biologically unpredictable tumours. The present study indicates that nearly half the cases could be classified as atypical or well-differentiated neuroendocrine carcinoma. However, there still remained some cases where the histology was typical but subsequent lymph node metastases developed.