Ultraviolet (UV) light therapy was given over a period of several weeks to 19 test persons on one side of the back. The duration of the treatment was gradually increased from 30 sec to 7 min. After conclusion of the treatment quantitative lipid determination was made on both the treated and the untreated sides of the back, using the direct extraction method and the paper absorption method. The lipids obtained in this way were analysed by thin-layer chromatography. The following were the most important results: (1) The amount of skin surface lipids was significantly increased by UV irradiation. This results goth from an increase in the epidermal lipids as well as in the lipids of the sebaceous blands. (2) The percentage of free cholesterol in the skin surface lipids was significantly increased by UV irradiation. This increase is probably connected with a thickening of the stratum corneum. (3) The percentage of free fatty acids in the skin surface lipids is significantly increased by UV therapy. This shows that the living conditions of the saprophyte skin flora, which set free upases, are improved as a result of this treatment or that this treatment leads to an increase of the activity of the bacterial upases in the excretory ducts of the sebaceous glands.