The distribution of allergic contact sensitization was assessed in an unselected population, living in western Copenhagen, Denmark. Ready-to-apply patch tests comprising 23 haptens and mixtures of haptens were mailed to 793 adults, and 567 (71.5%) participated. The tests were read 2 days after application. A total of 111 positive reactions were found among 86 (15.2%) subjects. Sensitization was less frequent in men than in women (11.5% versus 18.8%). Twenty out of the 23 chemicals in the test elicited positive reactions. Positive reactions to nickel and thiomersal were found most frequently (6.7% and 3.4%, respectively). Concerning the other chemicals the frequencies were 1.1% or less. Nickel sensitivity was less frequent in men than in women (2.2% versus 11.1%). The frequencies of sensitization probably represent minimum figures. Regulation of exposure needs to be considered in order to prevent primary sensitization and disease recurrences in those already sensitized.