['Iodopropynyl butylcarbamate (IPBC) is a new preservative in medical and cosmetic leave-on products. Although cases of allergic contact dermatitis to IPBC have been reported, it is not known whether the usual test concentration of 0.1% is appropriate for screening tests with IPBC.', 'To determine the concentration of IPBC that should be used in screening patch tests.', 'An analysis was made of data filed by 26 centres of dermatology on patch tests performed with one or two concentrations of IPBC (0.1%, 0.2%, 0.3% or 0.5%) in 8106 unselected patients. Criteria used to determine the best test concentration of IPBC were the reaction index, the positivity ratio, the rate of crescendo reactions, and the relations between IPBC reactions and the MOAHLFA index irritant reactions to sodium lauryl sulphate (SLS), and allergic reactions to other contact allergens including preservatives.', 'IPBC 0.1%, 0.2%, 0.3% and 0.5% yielded 0.5%, 0.8%, 1.3% and 1.7% positive reactions, but this increase was accompanied by an even greater increase in doubtful and irritant reactions. These figures and the other criteria examined suggested the range of suitable test concentrations of IPBC to lie between 0.2% and 0.3%. A detailed analysis of MOAHLFA indices and of associations between reactions to IPBC and reactions to other allergens and to SLS showed that most of the positive reactions to IPBC 0.2% can be assumed to be allergic ones and that with IPBC 0.2% fewer false-positive reactions can be expected than with IPBC 0.3%.'] Is iodopropynylbutyl carbamate 0.2 % suggested for patch testing of patients with eczema possibly related to preservatives?