Chronic granulomatous disease (CGD) is characterized by the failure of phagocytic leukocytes to kill certain bacteria and fungi. This is caused by deficiencies in one of the components of NADPH oxidase, the enzyme in phagocytic leukocytes that generates superoxide. In a rare, autosomal recessive form of CGD, a 67-kD cytosolic component of NADPH oxidase (p67-phox) is missing. Until now, mutations in the gene coding for this protein have not been identified. We now report on a 10-year- old girl with lymph node and liver abscesses who was recognized as an A67(0) CGD patient by lack of NADPH oxidase activity in her granulocytes, a cytosolic defect in a cell-free oxidase system, and lack of immunoreactive material with an antiserum against the p67-phox protein. mRNA for this protein was present in normal amounts in her monocytes. This p67-phox mRNA was reverse-transcribed, and the coding region was amplified by polymerase chain reaction in six overlapping fragments and was sequenced. The patient appeared to be homozygous for a G-233-->A mutation, resulting in a nonconservative amino acid change (78Gly-->Glu). This mutation was also found in the genomic DNA of this patient but not in that of 38 normal donors. Both parents and a sister proved to be carriers of the disease, as deduced from the mutation in only one allele. The carrier state was also manifested by intermediate superoxide production by their intact granulocytes and in the cell-free system.