Two cases of progressive multifocal leukoencephalopathy (PML) are reported in patients with psoriasis who were treated with fumarates, one with Fumaderm and the other with a compound containing dimethyl fumarate. The manufacturer of Fumaderm comments on the reports. To the Editor: Fumaric acid is considered effective and safe in the treatment of psoriasis vulgaris 1 and is licensed for this use in Germany. We diagnosed progressive multifocal leukoencephalopathy (PML) in a 74-year-old man who had received monotherapy for psoriasis with oral fumaric acid for 3 years (2007 through 2010) in doses of up to 120 mg of dimethyl fumarate and 95 mg of monoethyl fumarate, each taken two times a day. The patient had had psoriasis for more than 5 years and had been treated topically with glucocorticoids (January through May 2005) and orally with acitretin (maximum dose of . . .