Prolidase deficiency is a rare autosomal recessive disease characterized by various skin lesions, mental retardation and massive urinary excretion of imidodipeptides. We have been following and investigating sisters with this disease. Recently, two forms of prolidase were separated from most human tissues and cells. In prolidase deficient patients, the activity of prolidase I is remarkably reduced, but the activity of prolidase II is hardly reduced. This enzyme plays an important role in recycling of endogenous proline in situ especially for new collagen synthesis. Though the clinical manifestations of this disease have been attributed to the impaired recycling of proline, further investigation will be required to clarify the mechanism of development of clinical manifestations in this disease.