Dimethyl fumarate is a fumaric acid ester. It has been used for some years to treat psoriasis and also as a preservative in desiccant sachets in the transport of furniture and footwear. Its irritant properties and sensitizing potential in contact with the skin were recently highlighted when it was implicated as the causative agent in 2 epidemics of severe acute eczema: sofa dermatitis in northern Europe and shoe dermatitis in Spain. The present article aims to guide dermatologists in the diagnosis and management of patients allergic to dimethyl fumarate. We review the clinical manifestations, results of patch tests, possible cross-reactions, and sources of exposure to dimethyl fumarate responsible for these skin reactions.