There is a close connection between the killings of patients and the systematic murder of European Jews. Even shortly after the start of the war in 1939, SS units in Poland targeted Jews with disabilities or mental illness. The first mass murder of Jews in the German Reich took place in 1940 as part of the »Aktion T4«: in a special campaign, more than 2,000 Jewish psychiatric patients were killed solely because of their »race«. With the »euthanasia« murders extended as of the spring of 1941 to include concentration camp prisoners who were sick or unfit to work (»Aktion 14f13«), doctors deliberately selected Jewish prisoners.

»Aktion T4« served as a model for the »final solution to the Jewish question«. The killing technology developed for the T4 killing centres, the assembly-line-like killing procedure and the T4 personnel were all adopted to exterminate the Jews. As of the end of 1941, the »Chancellery of the Führer« assigned about 120 T4 men to carry out the murder of at least 1.6 million Jews in occupied Poland, known as »Aktion Reinhardt«. On account of their experience with mass gassings, they held key positions with regard to the establishment and organisation of the three extermination camps at Bełżec, Sobibor and Treblinka. All commandants of these camps, including Irmfried Eberl, the doctor and head of the T4 killing centres of Brandenburg an der Havel and Bernburg, belonged to T4. After the end of »Aktion Reinhardt«, around one hundred T4 staff were employed in the persecution of Jews and partisans in Italy from 1943 onwards.

Besides the personnel and organisational connections between the two programmes of murders, there is also an ideological one: the National Socialist racial ideology was based on the belief in human inequality and the targeted persecution and extermination of people regarded as inferior.