Patch tests with aqueous solutions of the irritating detergent sodium lauryl sulfate (SLS) elicit varying degrees of inflammation from subject to subject and from site to site. For an investigation of the causes of this variability, two patch tests with 10% aqueous solutions of SLS were applied to adjacent areas of ventral forearm skin of eighteen volunteers. In one test the water vehicle was allowed to evaporate from the patch test unit before the patches were applied. After 22 hours the patch tests were removed, and 2 hours later the degree of inflammation was graded. Less inflammation was present at the site of the dry patch test in fifteen of eighteen subjects, and the score of inflammation between each pair was significantly less at the dry patch test site (p < 0.001). These studies show that evaportion of water from aqueous solutions can influence the irritating potential of SLS on human skin.