Background: Methyldibromoglutaronitrile (MDGN) is a component of Euxyl K400, a preservative used in many skin care products in Europe. MDGN has been used in skin care products in the United States for the last 5 years. Contact allergy from MDGN has been reported from Europe. Objective: The purpose of this study was to determine the frequency of MDGN as a sensitizer in patients undergoing routine patch testing. Methods: We reviewed the results in 163 patients who underwent patch testing during a 4-month period to determine the number who had any reaction to MDGN at two different concentrations (0.2% and 0.5%). Tests were graded with the use of the North American Contact Dermatitis Group criteria (0 to 3+), and readings were performed at 48 and 96 hours (all positive reactions were evaluated at a follow-up visit or by telephone interview). Results: In the 4-month period, 45 of the 163 patients showed some reaction (± to 3+) at one or more readings. Of these, the results for 23 patients were considered to be irritant false-positive reactions; for 3 patients, the results were classified as uncertain; and for 19 patients, the results were classified as allergic. Of these, the results for eight patients were of definite relevance; the results for five patients were of probable relevance, and the results for six patients were of doubtful relevance to the problem condition. Other positive patch tests to a variety of allergens were frequently seen in persons positive to MDGN. Conclusion: MDGN is a sensitizer in skin products and, with the increase of its use, should be considered in the patch test evaluation of patients with persistent dermatitis. Optimum patch test concentrations are yet to be determined. (J Am Acad Dermatol 1998;38:934-7.)