Prolidase deficiency is an autosomal recessive inherited disease characterized clinically by frequent infections, mental retardation, and various skin lesions. Fundamental treatments for these manifestations have not been established. We performed adenovirus-mediated gene transfer of human prolidase cDNA into fibroblasts from patients with prolidase deficiency. Infection with the adenovirus vector carrying human prolidase cDNA increased prolidase activity in fibroblasts up to approximately 7.5 times of that of normal control fibroblasts. This indicates the feasibility of adenovirus-mediated gene therapy to treat patients with prolidase deficiency in the future.