It was mainly adult psychiatric patients who had been in an institution for a long time who fell victim to »Aktion T4«. They were selected according to utilitarian considerations: classed as »useless«, they could not help with the work in the institution. As »disturbing«, they hindered the operation of the ward. If they were »in need of care«, they caused the nursing staff a lot of work. Classed as »incurable«, they were no longer of medical interest for the doctors. These evaluations of the patients obviously predominated among the T4 victims.

Minors and pupils in care were killed both in »children's wards« and within the framework of »Aktion T4«. An important factor for being selected was the so-called »ability to learn« (Bildungsfähigkeit) – and thus the assumed future »benefit« of the minors to society. If they were deemed »incapable of learning«, they were at great risk of being murdered.

From 1941 onwards, T4 doctors selected concentration camp prisoners who were Jewish, unable to work, or regarded as »asocial«. They were murdered in the killing centres of Pirna-Sonnenstein, Bernburg and Hartheim.
 
Patients in preventive detention were deliberately murdered at the start of »Aktion T4«. By decision of the Reich Ministry of Justice, as of 1943, the principle of »annihilation through work« was supposed to apply to these people. The Gestapo took delinquent persons classed as »irresponsible« to concentration camps.

As of 1943, the T4 headquarters sent sick and exhausted forced labourers from Eastern Europe to the so-called collective institutions, such as Hadamar, where they were to be murdered.