The preservative methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) is a well-known sensitiser and present in the Swedish baseline series since the 1980s. The proportions of MCI/MI are 3:1. MI alone has been used as a preservative since less than 10 years. This study was conducted on behalf of the Swedish Contact Dermatitis Research Group to evaluate inclusion of MI in the Swedish baseline series since the preparation of MCI/MI might fail to detect contact-allergic reactions to MI alone. Patients with suspected allergic contact dermatitis at 5 Swedish dermatology departments were consecutively patch-tested with MI 2,000 ppm aq and MCI/MI 200 ppm aq. The number of cases with exclusive contact allergy to MI varied between 0.8-4.2%. In total, 1.9% reacted exclusively to MI and not to MCI/MI. Due to the considerable frequency of contact allergy to MI not traced by MCI/MI, MI 2,000 ppm aq is included in the Swedish baseline series from January 2014. This corresponds to a dose of 60 mu g/cm(2).