Background Contact sensitization by ingredients in personal products is an important clinical problem. It is not clear how sensitization is induced by the generally low concentrations at which they occur but it might be the result of repeated exposure.Objectives To compare the strength of contact sensitization induced by a single exposure to 2,4-dinitrochlorobenzene (DNCB) (60 lg cm )2 ) or three repeated exposures to a subsensitizing dose (10 lg cm )2 ). Methods Two groups (n = 10) of healthy adult volunteers were randomized to receive either a single patch of DNCB 60 lg cm )2 or three once-weekly applications to the same site of 10 lg cm )2 DCNB. Four weeks after the last application, sensitization was quantified by measurement of responses (skinfold thickness) to a graded series of four challenge doses. Results All the volunteers were sensitized and the strength of the responses was virtually identical between the groups. Conclusions The same degree of sensitization was induced by three exposures to DNCB 10 lg cm )2 as by one exposure to 60 lg cm )2 of DNCB. Thus repeated exposure to low doses of contact sensitizers may increase the sensitizing potency. This must be taken into account in future risk assessments.