Chronic granulomatous disease (CGD) is a rare inherited disorder in which phagocytic cells are unable to generate superoxide anions. Patients with CGD are predisposed to recurrent bacterial and fungal infections because the superoxide-generating NADPH oxidase activity is needed for efficient killing of microbes. Among the at least 5 subunits creating a functional NADPH oxidase, a molecular defect located in any of the gp91phox, p22phox, p47phox, or p67phox subunits may cause CGD. In this study, 8 patients were diagnosed with CGD on the basis of clinical findings and absence of nitroblue tetrazolium reduction in phagocytes. Southern blot analysis, GeneScan, and direct sequencing were performed to define particular DNA mutations. Among 6 X-linked CGD (X-CGD) patients, 4 different mutations were identified in the X- -linked CYBB gene (encoding gp91phox) by direct sequencing. A novel missense mutation, located in the NADPH-binding region of gp91phox, was found in 2 brothers. One frameshift 1578delA, one splicing 252G->A mutation, and one partial gene deletion were also identified. The molecular defect in the NCF1 gene (encoding p47phox) was established in 2 patients. One was a .GT/.GT homozygote, the other carried, besides this GT deletion on one allele, a unique Phe118stop mutation on the other. In general, the X-CGD patients within the group followed a more severe clinical course than the patients with an NCF1 defect. However, the lack of a straightforward genotype- -phenotype correlation indicates that the clinical severity of CGD depends also on other antimicrobial host-defense systems.