In view of the increased use of dermatological preparations containing a high concentration of propylene glycol (PG) for use under occlusive dressings, patch tests were performed with pure PG in a group of 98 out-patients at a skin clinic. No indication of a possible allergic contact dermatitis to PG could be found in the history of any of these patients. In 11 of them, the patch test reactions after a 48-h application were positive. In eight cases the reaction was scored as one plus, in two cases as two plus, and in the remaining case a three plus reaction was observed. In view of the negative history of all patients, the observed reactions were considered to be of a primary irritant nature. Histological examination of the positive patch tests revealed some definite quantitative differences between one and two plus reactions on the one hand and a three plus reaction on the other. The findings suggest that the application of high concentrations of PG under occlusive dressings may give rise to skin reactions of a primary irritant character which may closely simulate an allergic reaction.