Prothrombin deficiency is a very rare autosomal recessive bleeding disorder associated with mild to severe bleeding symptoms. We identified this bleeding disorder in a US-born patient as due to prothrombin Puerto Rico I. Unlike other prothrombin deficiencies, prothrombin Puerto Rico I is a series of concordant polymorphisms found in people of Puerto Rican descent with a much higher frequency than those prothrombin deficiencies found in the general population. This case underscores the importance of family history in identifying rare bleeding disorders. Am. J. Hematol 2007. © 2006 Wiley-Liss, Inc.