The 67-kD cytosolic protein (p67-phox) is an essential component of the superoxide-generating system in phagocytes, and its defect is known to cause chronic granulomatous disease (CGD). We sequenced p67-phox cDNA from one of seven patients found in Japan and his parents. In the patient's cDNA, homozygous AG dinucleotide insertion at position 399 (or 401) was found together with three other homozygous substitutions in a coding region (A-542 to G, T-895 to C and A-983 to G) compared with the sequence reported for HL-60 cells. In cDNA from his parents, the AG insertion was found to be heterozygous. In contrast, the other three substitutions were found homozygously in his father's specimen and the latter two in his mother's specimen. The substitution of A-542 to G was heterozygous in his mother's cDNA. The AG insertion would induce a frame shift and bring about a stop codon at the position of 433. However, the other three differences would give insignificant changes for the protein function, if any, because the substitutions of A-542 to G and A-983 to G result in the conservative amino acid transitions, ie, Lys-180 to Arg and Lys-327 to Arg, and that of T-895 to C no amino acid change. Neutrophils from the patients completely lacked superoxide generating activity whereas those from his parents generated substantial amounts of superoxide anion upon stimulation. Thus, it is concluded that the AG dinucleotide insertion is responsible for the disease in this patient.