Gray platelet syndrome (GPS) is an inherited bleeding disorder characterized by thrombocytopenia and the absence of α-granules in platelets. Patients with GPS present with mild to moderate bleeding and many develop myelofibrosis. The genetic cause of GPS is unknown. We present 2 Native American families with a total of 5 affected persons and a single affected patient of Pakistani origin in which GPS appears to be inherited in an autosomal recessive manner. Homozygosity mapping using the Affymetrix 6.0 chips demonstrates that all 6 GPS-affected persons studied are homozygous for a 1.7-Mb region in 3p21. Linkage analysis confirmed the region with a logarithm of the odds score of 2.7. Data from our families enabled us to significantly decrease the size of the critical region for GPS from the previously reported 9.4-Mb region at 3p21.