The nationwide Swedish Family-Cancer Database of 9.6 million individuals was used to analyse the development of second neoplasia after 6909 thyroid and 12697 other endocrine tumours. Tumour cases were retrieved from the Swedish Cancer Registry from 1958 to 1996. The risk of a second endocrine tumour was markedly increased compared with first endocrine tumour; e.g. the standardised incidence ratios (SIRs) were well over 10 for adrenal tumours after thyroid cancer, and vice versa. Familial risks were higher for the development of second compared with first neoplasms, and the SIRs for men were usually higher than those for women. Many increases between different endocrine glands can probably be ascribed to known cancer syndromes. Even cancers at some other sites were increased after the development of primary endocrine tumours. Notably, small intestinal carcinoids were increased after thyroid and other endocrine tumours, and brain menigiomas were increased after parathyroid and pituitary adenomas. These novel associations suggest shared risk factors for these sites. However, many endocrine tumours are benign and the diagnosis of the first tumour may increase the likelihood of a second diagnosis.