The preservative methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) is a well-known sensitiser and present in most baseline series since at least 20 years. The proportions of MCI/MI are 3:1. MI alone has been used as a preservative in occupational and household products, and cosmetics since less than 10 years. MCI/MI tested at 100 ppm fails to detect a significant percentage of contact-allergic reactions to MI. Our aim was to investigate whether a separate test preparation with MI picks up additional cases of contact allergy to MI not detected with MCI/MI 200 ppm. MI was inserted into the baseline series of the Malmö clinic in 2003 starting at 475 ppm, then 900 ppm, then 1,000 ppm, 1,500 ppm and finally 2,000 ppm. In 5,881 consecutively tested dermatitis patients the contact allergy rate for MI varied between 0.5 and 6.5%, with a marked increase in recent years. The contact allergy rate to MI 2,000 ppm alone, not traced by MCI/MI 200 ppm, varied between 0 and 1.9 %. In conclusion, due to the increase of contact allergy to MI not traced by MCI/MI 200 ppm, MI in water at 2,000 ppm should be tested in a baseline series. Independent of patch test technique a dose of 60 µg/cm should not be exceeded to avoid adverse reactions and particularly patch test sensitisation.