An Egyptian Jewish family in which polysyndactyly is transmitted through four generations with 17 affected individuals is presented. The malformation is caused by an autosomal dominant gene with full penetrance and variable expressivity. In the newborn propositus the gene was fully expressed producing polysyndactyly of both hands and both feet. The mother had polysyndactyly of the feet but only syndactyly of the third and fourth fingers of the right hand and a post minimi on the left hand. In addition she had Marfan's syndrome due to a new mutation and transmitted Marfan's syndrome to one of her daughters who did not have polysyndactyly. Apparently the association of Marfan's syndrome with polysyndactyly has not been previously described.