From February 1989 to January 1990, the Swiss Contact Dermatitis Research Group conducted a 1-year study to examine the frequency of sensitization to a series of 13 common preservatives. A group of 2295 consecutive outpatients with suspected allergic contact dermatitis (age range 7-90 years, with a mean age of 42; 911 males, 1384 females) was tested. The %s of positive reactions to the preservatives studied are as follows, in descending order: formaldehyde 5.7%, benzalkonium chloride 5.5%, Kathon CG 5.5%, thimerosal 4.2%, chlorhexidine digluconate 2.0%, DMDM hydantoin 1.7%, paraben mix 1.7%, chloroacetamide 1.5%, Bronopol 1.2%, imidazolidinyl urea 1.0%, quaternium 15 1.0%, triclosan 0.8%, 2,4-dichlorobenzyl alcohol 0.4%. These relatively high values suggest a heavy exposure of the Swiss population to topical preservatives. Compared to previous studies, the sensitization rate to Kathon CG has stabilized in Switzerland over the last 2 years. Sensitization to formaldehyde portrayed impressive geographical variation, with sensitization rates up to 9% in western and only 3% in eastern Switzerland. The low sensitization rate to parabens argues for their inclusion in a medicament or preservative series, rather than in the standard series.