In a kindred with a mild, recessively inherited variant of the Ehlers-Danlos syndrome (EDS), a platelet aggregation defect segregated concordantly with skin and joint abnormalities. This defect was partially corrected in vitro by addition of normal plasma or cryoprecipitate. The plasma of the patients with EDS failed to support the aggregation of normal gel-filtered platelets in response to collagen; this defect was completely corrected by the addition of normal human fibronectin. Since fibronectin is an important adhesive glycoprotein in connective tissue and is required for normal platelet interactions with collagen, we propose that both platelet malfunction and joint hypermobility in this kindred are likely explained by a defective fibronectin.