Preservatives used in cosmetics tend to be, by their nature, allergenic. Methylisothiazolinone (MI) has been used as a sole preservative in multiple cosmetics, household goods, and toiletries. A current epidemic of MI has recently been reported in Europe.The aim of this study was to study the prevalence of methylchloroisothiazolinone/MI (MCI/MI) and MI allergy in a Bangkok dermatology clinic.During January 2009 to June 2014, 3253 consecutive patients tested with 100 ppm (0.01%) MCI/MI and patients tested with 2000 ppm (0.2%) MI were included in the study.Three hundred twenty of 3253 patients (9.8%) tested for MCI/MI had a positive reaction. There was a steep increase in the prevalence of MCI/MI contact allergy from 4.8% in 2009 to 11.2% in 2011 and 17% in 2013. In the first 6 months of 2014, 22 of 54 cases tested for MI (40.7%) had a positive reaction. Among those who had a positive reaction to MI, 6 of 22 (27.3%) showed negative reaction to MCI/MI.There is a similar prevalence of MI allergy in a Bangkok dermatology clinic as those reported in European centers such in the United Kingdom.