There has been a hypothesis that Hodgkin's disease in young adults and multiple sclerosis may have related causes because the age of clinical onset and the geographic distribution of both are similar. This hypothesis was tested for data in Denmark. Between 1943–62, the average annual incidence rate for Hodgkin's disease in Denmark was 2.25 per 100,000 population (2.68 male and 1.83 female). Between 1951–69, the average annual death rate for Hodgkin's disease was 2.15 per 100,000 (2.66 male and 1.64 female). the average annual incidence rate for multiple sclerosis in Denmark was calculated from age at onset for 2,481 prevalent cases of 1949, the 1940 population, and an average annual incidence of 128,86 cases for 1939-45: the average annual incidence rate per 100,000 was then 3.35 (3.00 male and 3.69 female). Age specific incidence and death rates for Hodgkin's disease in Denmark each showed a bimodal curve, with one peak at age 25–29 and the other at age 70–74; this was found for each sex, with male rates consistently higher than female. the age specific incidence rates for multiple sclerosis were clearly unimodal with a peak at age 25–29 more definite in females than males. Rates for MS were notably higher for young females than males but about equal by sex for those over the age of 30. the geographic distribution of multiple sclerosis within the counties (amter) of Denmark was markedly non-random, with the major concentration of high prevalence areas across middle Jutland and on to Fyn. Geographic distribution of Hodgkin's disease, whether for the young or the old, and whether from incident or death cases, showed no significant variation from a homogeneous distribution. In formal testing there was no correlation of any Hodgkin's distribution with that of MS. A review of the Hodgkin's data for distribution in the United States, on which the original hypothesis was based, suggests the variation there may be little more than reporting artifact. Accordingly, we conclude that there is no relation between distributions of these two disorders, and the factors they do appear to have in common are either quite non-specific or of questionable validity. Thus there is no reason to believe that multiple sclerosis and Hodgkin's disease, even in the young, share a common etiology.