SummaryBackgroundMethylchloroisothiazolinone (MCI)/methylisothiazolinone (MI) and MI are common preservatives in industrial and household products and in cosmetics. Occupational contact allergy to MCI/MI or MI has been reported among painters and paint factory workers in particular.ObjectivesTo retrospectively describe patients with occupational contact allergy to MCI/MI and MI at the Finnish Institute of Occupational Health (FIOH).MethodsPatch test files and clinical records of patients examined at the FIOH between January 2002 and February 2013 were reviewed to identify the patients.ResultsSeventy‐two of 1745 patients showed positive reactions to MCI/MI and/or MI. Of these, 36 were occupational cases, and 31 were diagnosed during the second half of the study period. Hairdressers and beauticians, mechanics and repairmen, and machinists were the most significant occupational groups, followed by painters and paint factory workers, and café and restaurant workers.ConclusionsWe observed a sixfold increase in the number of occupational cases in the second half of the study period as compared with the first half. Liquid soaps and industrial hand cleansers were common sources of exposure to MCI/MI or MI. A total of 33% of the patients used products without any mention of MCI/MI or MI in safety data sheets or product declarations.