Several members of a family living on the west coast of Scotland and on one of the islands off the coast had serious thrombotic disease. The plasma antithrombin III (ATIII) concentrations were measured by both functional and immunological assay in all available members of the family. Concentrations were 25% to 66% of normal in 12 people, including all seven with thrombotic disease. The inheritance pattern was characteristic of an autosomal dominant disorder. Thrombotic disease generally affected the leg, mesenteric, and axillary veins, although one man who had died before the study began had had severe arterial atheroma. In women the first thrombotic symptoms usually occurred during pregnancy. None of these patients have developed thrombotic symptoms until they were at least 18, so four younger members of the family who have ATIII deficiency but no thrombotic disease may eventually develop symptoms.