The clinical course of sarcoidosis is in many cases prolonged, and may be preceded by a period of varying duration during which granulomatous changes are present in various organs without causing symptoms; and it is certain that some individuals go through the entire course of sarcoidosis to spontaneous resolution without, or with only trivial symptoms, although granulomas if sought would be found in involved organs. Thus it is to be expected that a proportion of patients will show evidence of some other disease during the course of known sarcoidosis; and also that during the investigation of patients found to be suffering from some other disease, evidence of previously unsuspected sarcoidosis will be found occasionally. The associations of greatest interest are those that may cause diagnostic difficulty because one disease masks the other; and those in which there is some possibility of a pathogenetic relationship through a common immunological, genetic or other factor between sarcoidosis and the other disease, or through some feature of sarcoidosis that predisposes to the other disease. If such a relationship seems possible, it is important to determine whether the association occurs more frequently than would be expected by chance. Because of uncertainty about the total incidence of sarcoidosis, including undiagnosed and asymptomatic cases, estimates of the expected frequency of associations with other diseases are subject to considerable error.