The purpose of this in vivo study was to investigate, non-invasively on human subjects, xerotic skin and its physiological evolution over time, compared to normal skin. Two groups of 17 female subjects were studied during the winter season, one made up of subjects with normal skin and the other subjects with xerotic skin. A clinical assessment and biometrological measurements of hydration and transepidermal water loss (TEWL) were performed on the same area of the external antero-lateral surface of the leg at the start of the study then after three weeks. At the end of the study, the ultrastructure of stratum corneum samples taken from the same area was examined by transmission electron microscopy. Subjects with xerotic skin were selected according to their impaired cutaneous barrier function, reflected in a TEWL higher than 12 g/m ; 2/h. Compared to normal subjects, they presented a hydration level more than 25% lower. After an interval of 21 days, no significant change in the hydration level or clinical appearance of the xerotic skin was observed. In contrast, the TEWL had decreased significantly (D _ 21- D _ 0=-3.6 g/m ; 2/h; p < 0.001) but still stayed higher than normal values. Changes in the ultrastructure of the stratum corneum were also observed in the subjects with xerotic skin. Unlike normal skin, corneosomes could be detected right up to the surface layers, accompanied by intercellular lipids in an amorphous form. These observations confirm the important roles played by both corneosomes and lipid organization in the cohesion/desquamation processes. In the subjects with normal skin, the hydration level and barrier function remained unchanged during the three week study but an onset of skin dryness was observed, the mean clinical score increasing by +1.3 (p = 0.01). These results confirm that there is no direct relationship between TEWL and the severity of skin dryness. It appears that a clinical evaluation is more sensitive than biometrological measurement for describing early state of cutaneous dryness. This study highlights the importance of a regular cosmetic or dermopharmaceutical treatment during the winter to prevent xerosis apparition on legs.