SinmaryRelatively little is known about the epidemiology of carcinoid tumours in contrast to the extensive information available on their biochemical effects and natural history. Accordingly, we have used cancer regtstrations in England from 1979 to 1987, and in Scotland from 1980Scotland from to 1989 Carcinoid tumours were so named by Oberndorfer in 1907 because they resembled carcinomas but were thought to be of a more benign nature (Grahame-Smith, 1972). Since then the malignant potential of these tumours has been recognised. MacDonald (1956) has suggested that all extra-appendiceal carcinoids should be considered potentially malignant. Carcinoid tumours are the most common tumour of the appendix, the most common gastrointestinal neuroendocrine tumour and the most common form of bronchial adenoma. They may present with non-specific abdominal symptoms, with local symptoms such as haemorrhage or obstruction of the bowel or a bronchus, with evidence of metastasis or with the malignant carcinoid syndrome. This syndrome, first described in 1934, is characterised by facial flushing, bronchoconstriction, episodic diarrhoea and right-sided valvular heart disease (Cassidy, 1934;Grahame-Smith, 1972 England Cancer registration data were obtained from the OPCS on all carcinoid tumours first registered in 1979-87 in English residents. Although the OPCS also holds Welsh data, these could not be used in this study because they rarely include tumour morphology codes. Registrations of carcinoid tumours were identified using morphology codes 8240-8244 from the International Classification of Diseases for Oncology (ICD-O) (WHO, 1976). The tumours had been given either malignant site codes (ICD-9 140-208) (WHO, 1977) or site codes which indicated that their future behaviour was uncertain (ICD-9 235-238). Before 1979, when ICD-8 (WHO, 1%9) was used, tumours of uncertain behaviour were not coded separately from benign tumours and were not recorded by cancer-registries. The majority of carcinoid tumours are of uncertain malignancy. This study, therefore, was restricted to the period after 1979 when only tumours specifically reported as benign by the pathologist would not have been registered.Registrations were subdivided according to tumour site using the ICD-9 classification. Where necessary individual site codes were aggregated as follows: 'gastrointestinal tumours' ICD-9 150.0-154.8 and 235.2; 'thoracic tumours' ICD-9 162.0-165.9, 235.7 and 235.8. The uncertain codes are less specific with respect to site than the malignant codes (see Table I). The natural history of carcinoid tumours is such that the distinction between a malignant tumour and one of potential malignancy is not easily made. On examination of the data, there seemed to be inconsistency among registries in the proportion of cases assigned a malignant, as opposed to an uncertain, site code. Because of this evidence of crossover between the categories we did not analyse malignant tumours separately from those of uncertain behaviour.As the coding of carcinoid t...