We report three kindreds in whom plasminogen deficiency was associated with thrombosis and in whom the ratio of functional and immunological values of plasminogen was consistent with type I deficiency. An additional subject with plasminogen deficiency is also described. The three propositi presented with venous thrombotic disease. The fourth subject presented with a thrombotic stroke. Investigation of family members in three of these four cases revealed other subjects who were found to have low levels of plasminogen and who were asymptomatic. The pattern of inheritance appears to be autosomal dominant. In one woman, plasminogen levels were shown to rise to within the normal range during pregnancy and returned to low levels after delivery. A total of eight pregnancies were reviewed in our series and no thrombotic events occurred.