Hans Roemer (1878-1947), director of the psychiatric asylum in Illenau/Baden, was one of the few psychiatrists during National Socialism who explicitly turned against the program of systematic patient killing (euthanasia). The article gives an overview of Roemer's biography and sketches his activities in the context of reform movements in the 1920s as a protagonist in the context of the German mental hygiene movement and propagator of eugenic measures including the Nazi law of compulsory sterilization of those classified as suffering from hereditary conditions. The focus is on the beginning of the "Aktion T4", the first phase of the systematic patient killing in 1939/1940. Roemer repeatedly tried to avert the deportation of patients from the Illenau and to find psychiatric allies for coordinated steps against the killing program. Ernst Rüdin, president of the German psychiatric association, as well as leading medical administrators blocked Roemer's initiatives. Nevertheless, Roemer did not experience any harassment or sanctions by representatives of the regime.