Carcinoid tumours of the lung are commonly misdiagnosed as carcinoma and even if they are recognized their behaviour may be unexpected. In a series of ten cases which had caused diagnostic difficulty there had been an incorrect interpretation of broncial biopsy material in four, of the resected tumours in another four, of a needle biopsy core in one and of autopsy material in another. In some cases these errors led to needless radiotherapy and to an unnecessarily poor prognosis being given. In all cases the diagnosis of carcinoid tumour was made retrospectively by the demonstration of argyrophil granules in the tumour cells or by electron microscopy. Electron-dense bodies could be found in material that had been fixed in formol saline or even embedded in paraffin so that this method may be used retrospectively. Two of these ten tumours behaved in a malignant fashion but progressed much more slowly than most samples of bronchial carcinoma. The possibility of carcinoid tumour should be considered in any patient with a tumour of the lung who is unusually young, or who is a non-smoker or where bone or calcification are present in the tumour. It is also suggested that unrecognized cases of carcinoid tumour may be found amongst long-term survivors with lung tumours which havebeen treated by radiotherapy or chemotherapy without surgery.