1. A case of congenital deficiency of proconvertin in a Hopi Indian boy is reported. This is the eleventh case of "pure" proconvertin deficiency and the second reported in the American literature.
2. Laboratory studies revealed normal intrinsic blood coagulation but markedly abnormal tests when coagulation was studied in the presence of tissue extracts.
3. An accompanying mild deficiency of prothrombin was suggested by the results of some tests.
4. The concept that the role of proconvertin in hemostasis is primarily as a co-thromboplastin is supported by the clinical and laboratory studies carried out on this patient. The differentiation of this disorder from other cases with deficiencies of "serum" factors is discussed, and the previously reported cases are reviewed.