During the period 1962-71 a total of 2544 patients with respiratory sarcoidosis were reported to the Danish Institute of Clinical Epidemiology. Among them 48 patients developed a malignant tumour, the follow-up period ending on 31 December 1971. Only 33·8 cases of cancer were expected if sarcoidosis patients had had the same rates as the general population; the difference between the expected and observed number is statistically significant (0·02 > P > 0·01). Malignant lymphomata occurred 11 times and lung cancer 3 times more frequently than expected. For all other forms of cancer taken together, there was no significant difference between the expected and the observed number of cases.The increased cancer incidence may result from immunological deficiencies in patients with sarcoidosis.