Progress in development of gene therapy for chronic granulomatous disease (CGD), an inherited defect in leukocyte oxidase deficiency, is reviewed. The use of retrovirus vectors to transfer oxidase enzyme subunit cDNA sequence into hematopoietic progenitors results in correction of oxidase activity in neutrophils differentiating from transduced progenitors. In CGD mouse knockouts (X-linked gp91phox-deficient CGD and autosomal recessive p47phox-deficient CGD), gene therapy correction of the CGD defect resulted in appearance of oxidase-normal neutrophils in the peripheral blood and increased host resistance to challenge with fungi or bacteria. In a phase I clinical trial of ex vivo gene therapy of p47phox-deficient CGD, prolonged production (2-6 months) of a low number (1:5000) of oxidase-normal neutrophils was achieved. This therapy might prove beneficial in a setting of prolonged infection in CGD patients, in which even transient production of autologous gene-corrected neutrophils might augment host defense.