Allergic contact dermatitis is recognized as a public health problem and some major allergens have been subject to intervention aiming at lowering skin exposure. There is an obvious interest in evaluating the effect of such interventions. Population studies are difficult to perform and epidemiological studies based on clinical data from testing patients with contact dermatitis are common surrogates. Our objective was to gather Swedish clinical standard series test data on two occasions in order to monitor trends in sensitization rates. Consecutive patch test results from the Swedish standard series were collected from 9 centres from 1991 to 1993 and from 1999 to 2001. In total, 3680 and 3790 patients, respectively, were included. Crude, age-adjusted and age-stratified prevalence are given separately for women and men. Our top 10 allergens are much in line with newly published European test data. Significant changes among those allergens are increasing sensitization rates for Myroxylon pereirae and decreasing rates for colophony, 5-chloro-2-methyl-4-iso-thi-azo-lin-3-one, Amerchol L 101 and thiuram mix. Nickel allergy is decreasing among young women. Among less common allergens, a noteworthy increase of sensitization to 4-phenyl-en-diamine is found. In conclusion, significant trends in sensitization rates of important allergen, reflecting changes in exposure, have been found.