Systemic and sometimes topical therapy with fumaric acid (FA) and its derivatives is used in the treatment of psoriasis. Scattered data show that the topical application of these derivatives elicits side effects. Application of FA and some derivatives on the skin was accompanied by perilesional skin irritation, macular papular rashes and urticarial reactions. In order to determine the irritating and sensitizing properties of FA derivatives we used a cytotoxicity, flank irritation, ear swelling and guinea pig maximization test. The results of the cytotoxicity test demonstrated that dimethylfumarate (DMF) was the most toxic derivative. DMF induced also contact-urticarial reactions in contrast to mono-ethylfumarate (MEF). Challenge experiments with FA, MEF and DMF in MEF- and DMF-sensitized guinea pigs demonstrated that both MEF and DMF are moderate contact sensitizers. In DMF-sensitized animals cross-reactions with MEF were found. As DMF and MEF have cytotoxic, contact-urticarial and/or sensitizing properties, topical application should be avoided.