The role of regular skin cleansing with soap or syndet in pre-acne is ill-defined. The intention of this study was to assess the relative value of an acidic syndet bar and a conventional soap bar in the prevention of acne lesions in acne-prone patients. In a randomized, open, comparative trial the three months' application of either an acidic syndet bar or a conventional soap to facial skin for 1 min each in the morning and in the evening was compared in 120 adolescents and young adults with inflammatory acne grade I or II according to the Plewig and Kligman classification. It was a confirmatory trial with the number of inflammatory lesions being the prime parameter of concern. In addition, non-inflammatory acne lesions were analyzed as were parameters of safety such as itching, redness and scaling. While the number of inflammatory acne lesions, i.e., papulopustules, did not differ in the two trial groups composed of 57 evaluable cases each, this was the case from 4 weeks of application onward: in the group using soap the mean number of inflammatory lesions increased from 14.6 (+/- 5.3) to 15.3 (+/- 6.0), while it decreased in the other group from 13.4 (+/- 5.2) to 10.4 (+/- 5.8) (p < 0.0001). Symptoms or signs of irritation were seen in 40.4% of individuals belonging to the former and 1.8% belonging to the latter group. The number of papulopustules characteristic of inflammatory acne thus is clearly lower when a syndet bar of the acidic type is regularly used for cleansing the face as compared to a (necessarily alkaline) soap.(ABSTRACT TRUNCATED AT 250 WORDS)