A family is described in which a syndrome resembling moderately severe classic hemophilia was apparently inherited as an X chromosome-linked trait. In two affected individuals, the titer of functional antihemophilic factor varied dramatically from time to time, while the conversion of prothrombin to thrombin was impaired in no apparent relationship to AHF functional activity. A transfusion of 200 ml of fresh-frozen plasma did not correct the serum prothrombin times in either patient. In vitro, the additions of 10% of normal plasma or serum or washed plain or frozen platelets also did not normalize the serum prothrombin times. No inhibitor could be demonstrated in the blood of either patient. In one patient, RH, dissipation of infused cryoprecipitated AHF was abnormally slow, and, after an intensive course of transfusion of cryoprecipitate and whole blood, the titer of functional AHF remained at normal levels for at least 1 wk. The plasma of RH inhibited a human antibody against AHF in proportion to its titer of functional AHF (i.e., the defect was CRM-) despite the presence of relatively greater amounts of antigenic material recognized by heterologous antiserum. No qualitative abnormality of the AHF-like material in RH's plasma was identified. Inheritance of the abnormality appears superficially to be X chromosome-linked; on this assumption, three of four obligate carriers of the disorder were recognized by the presence of excess amounts of AHF-like antigens relative to AHF functional activity. This coagulation disorder has been designated Heckathorn's disease and may presage the discovery of other examples of hemophilia-related syndromes.