At eight dermatological clinics, a total of 2146 patients were tested between May 1990 and December 1991 to determine the significance of the preservative benzalkonium chloride in inducing allergic contact dermatitis and to evaluate recommendations for patch testing. In 225 cases an allergic reaction was proven, with 258 irritant reactions in addition. Only 12 cases were clinically relevant. Therefore, benzalkonium chloride is considered a weak allergen. A doubtful (erythema) or an incipient positive (with occasional papules) reaction may be regarded as an irritant reaction. In isolated cases a "repeated open application test" (ROAT) should be applied to determine clinical relevance. Our recommendation for patch testing is 0.1% benzalkonium chloride in vaseline, and also in water for special cases.