We have carefully examined four patients with desmoid tumor (DT) and their 31 relatives. In three of four cases, biopsies of the DT demonstrated low yet significant amounts of estrogen but not progesterone receptors in the tumor cytosol. In the fourth case, where the receptors were not demonstrable, the affected patient was a menopausal woman and the receptors may have been blocked by endogenous estrogen. Fourteen of their 31 relatives demonstrated multiple minor bone malformations in x-ray screening of the skeleton. The inheritance of these malformations was compatible with an autosomal dominant trait with variable penetrance. These findings are compatible with our suggestion that the basic underlying cause for DT is an inherited defect in growth regulation of the connective tissue. When a trauma is superimposed on such an individual, a DT may result. The growth of the tumor is, however, controlled primarily by sex hormones, estrogen predominance over progesterone being inducive to tumor growth.