Among 72 patients with the Peutz-Jeghers syndrome malignant tumours have developed in 16 (22%) of whom all but one have died. There were nine gastrointestinal and seven nongastrointestinal tumours. The relative risks of death from gastrointestinal cancer and all cancers were 13 (95% CI 2.7-38.1) and 9 (95% CI 4.2-17.3) respectively. The chance of dying of cancer by the age of 57 was 48%. There is evidence for a hamartoma/carcinoma sequence in the Peutz-Jeghers syndrome, suggesting that the gene locus involved is relevant to the development of malignancy in general.