In the early 2000s the preservative methylisothiazolinone (MI) was released as an individual preservative for industrial products and, in 2005, it was permitted for use in cosmetic products. Up until then MI had been used only in combination with methylchloroisothiazolinone (MCI). MCI/MI is one of the most frequent causes of preservative contact allergy and early studies showed that both MI and MCI are sensitizers. The prevalence of MI contact allergy is already around 1·5% and sources of exposure are associated with occupation, cosmetic products or household products. Use of MI in industrial products is not restricted and cases of occupational contact allergy to MI, e.g. in painters, are reported. The frequency of use of MI in cosmetics is low, around 1%, while up to 16·5% of household products were preserved with MI. We found 19 (1·5%) out of 1272 cosmetic products labelled with MI, primarily rinse-off products, and analysed the concentration of MI by high-performance-liquid-chromatography the ultraviolet and mass spectrometry detection. The use concentration ranged between 2 and 100 ppm. Repeated exposure to MI showed that many patients allergic to MI reacted to 50 ppm which is half the maximum permitted concentration of MI in cosmetics. The recent cases and prevalence studies on MI contact allergy could be the first sign of an epidemic of MI contact allergy. The development in prevalence of MI contact allergy should be closely monitored by including MI in the European Baseline Series at 2000 ppm.