Summary The medical and biological literature was reviewed with stress laid on the role of pyrrolidone carboxylic acid (PCA) and its sodium salt (NaPCA) in skin, its metabolism, its functions. The paper also includes a summary of 8 years of evaluation work carried out in our Laboratory on creams and lotions containing PCA-NaPCA which were assessed by biophysical (impedance measurement, alpha relaxation) and clinical methods. It is now definitely demonstrated that PCA is an hydrating agent and that all the cosmetic preparations containing at least 2% of the PCA-NaPCA salt system improve the condition of dry skin at short or long term provided an adequate vehicle is used (e.g. aqueous solutions are ineffective). The mecanism of action is discussed with reference to metabolism and physiological role of PCA in stratum corneum.