Absolute numbers of lymphocytes and dose of fumaric acid. After two months of application the lymphocyte count fell to a level of 700 cells/μl. At that point treatment was stopped until lymphocyte count recovered to a number of 900/μl after two and a half months. Dimethyl fumarate was given in an increasing dose up to 360 mg per day. When the symptoms of the psoriasis remained stable, the daily dose was decreased to 240 mg and increased again when the symptoms remitted. At the same time lymphocytes remained at an average level of about 600/μl. When they first fell under 500 cells/μl in October 2014, dimethyl fumarate dose was reduced to 120 mg. Finally, the therapy was stopped by the diagnosis of a PML. During the following months lymphocytes recovered slowly and reaching a number of 3000 – 5000 cells/μl.