Background: Methylisothiazolinone (MI) is used as a preservative in occupational, domestic products and, since 2005, in cosmetics. It is a part of the preparation of methylchloroisothiazolinone (MCI)/MI and only in the last years started to be tested as a single allergen in the baseline series. According to recent studies, the sensitization rate for MI varies between 1 and 6%, with a marked increase, particularly in the late years. Objectives: To describe patients sensitized to MI and MCI/MI and investigate its prevalence among patch tested patients Methods: Cross-sectional retrospective study, including all patients who performed, from 2011 to 2016, the European and Portuguese baseline patch test series, in a Dermatology department of a tertiary hospital. Those positive to MI and/or MCI/MI, were selected. Results: During the study period, 1768 patients (70% women), performed the baseline series; 972 (55%) tested positive according to the International Contact Dermatitis Research Group's scoring system. Of these, 114 individuals (11.7%) presented positivity to MI and/or MCI/ MI; 81 (71%) were women; mean age (minimum-maximum) of 44.3 (8-86) years; 33.3% had previous history of atopic comorbidities. Hands either isolated or associated with other body parts were the most frequent symptomatic location (54 cases, 47.4%) followed by generalized dermatitis (33 cases, 29%). Further comparison of MI sensitization revealed no significant differences with respect to gender (p = 0.267), age (p = 0.616), or the presence of allergic comorbidities (p = 0.536). In 2011, only one patient (0.3%) tested positive to MCI/MI and after June 2012 patients (1.1%) were positive to MI. After 2012 we observed a significant increase in