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Background Methylchloroisothiazolinone ( MCI) and methylisothiazolinone ( MI) are the active ingredients in commonly used preservative systems (e.g. Kathon CG®). MCI/ MI is present in the European baseline patch test series at 100 ppm aq. Since 1986, 200 ppm (dose 0.006 mg/cm2) has been used in Sweden without causing skin irritation. Centres in Spain, the United Kingdom and Ireland have also used 200 ppm in their baseline series. Objectives To find the optimal patch test concentration for MCI/ MI. Materials and methods MCI/ MI 100 ppm aq. and MCI/ MI 200 ppm aq. were simultaneously patch tested in 3300 consecutively tested dermatitis patients at eight European patch test clinics and one US patch test clinic. With the Finn Chambers® technique (diameter 8 mm), 15 µl was micropipetted on to the filter paper in the chamber. The corresponding volume for Van der Bend® chambers was 20 µl, and that for IQ Chambers® was 25 µl. Results Contact allergy to MCI/ MI at 100 and 200 ppm was found in 1.2% and 2.1% of patients, respectively ( p < 0.001). Conclusions MCI/ MI 200 ppm aq. (dose 0.006 mg/cm2) diagnoses significantly more contact allergy than the presently used concentration of 100 ppm (dose 0.003 mg/cm2), without resulting in more adverse reactions. MCI/ MI at 200 ppm should therefore be considered for inclusion in the European baseline test series. [ABSTRACT FROM AUTHOR]