One hundred multiple sclerosis (MS) patients and hospital controls were compared for coexisting illnesses, and other illnesses among their family members. The patients and controls were compared for allergy, migraine, and cancer among themselves and for a family history of these illnesses. The findings were negative. However a relationship was found between multiple sclerosis and diabetes mellitus. More MS patients than controls were either diabetic or reported a blood relative with diabetes. MS patients who reported another blood relative with MS were also more likely to report a family history of diabetes mellitus than MS patients who had no other relative with multiple sclerosis.