SummaryBackgroundThe prevalence of contact allergy to the isothiazolinone preservative methylchloroisothiazolinone (MCI) in combination with methylisothiazolinone (MI) and MI alone has increased in the last couple of years.ObjectivesTo investigate the prevalence of contact allergy to MI, MCI/MI and benzisothiazolinone (BIT) among patch tested patients at Gentofte Hospital, as well as the use of MI in cosmetic products.MethodsPatients patch tested with either MI, MCI/MI or BIT from 2010 to 2012 were included in the study. The MOAHLFA index was registered in all patch tested patients, and relevant exposures were determined in patients with an isothiazolinone allergy. In a market survey, the ingredient labels of cosmetic products were investigated for MI content.ResultsThe prevalence of MI and MCI/MI contact allergy increased significantly from 2010 to 2012: from 2.0% to 3.7% for MI (n = 2766), and from 1.0% to 2.4% for MCI/MI (n = 2802). MI‐allergic patients had occupational, hand and face dermatitis significantly more often, and were aged > 40 years. Cosmetics were the most common substances causing relevant exposure found in both MCI/MI‐allergic and MI‐allergic patients. MI was found in 3.3% of cosmetics on the Danish retail market.ConclusionsThe increase in MI contact allergy is alarming, and urgent action is needed.