[1] As a police psychiatrist, he started the first observation department for psychiatric patients in an effort to avoid having to put them in prison while they were waiting for an ordinary hospital place.

[1][4] Langfeldt was a keynote speaker at the 2nd International Congress for Psychiatry held in Zürich in 1958 devoted to knowledge on "groups of schizophrenia".

[3] Langfeldt also published several books on psychological themes for the general public, among them Nervøse lidelser og deres behandling (Nervous Diseases and Their Treatment), Hvorfor blir et ekteskap ulykkelig?

[1] In October 1946, Langfeldt was assigned the task of making a judicial observation of Norwegian author Knut Hamsun, who had actively supported the Nazi regime during the German occupation of Norway.

[6] In 1949, Hamsun published Paa gjengrodde Stier (On Overgrown Paths), a mixture of self-biography and storytelling,[9] covering the period from when he was arrested in 1945 to the Supreme Court verdict in 1948.

[6] As On Owergrown Paths was considered to have good literary standard, the book raised questions on whether Langfeldt and Ødegård had been correct in their diagnosis, though most psychiatrists agreed with them.

Compared to the original medical evaluation, Langfeldt in 1952 put more emphasis on organic brain disease than on pathological character traits.

[5] Some critics of the diagnosis argued that it might have been influenced by the Norwegian government,[6] which didn't want to see Hamsun in prison due to his advanced age and high status as a writer.

A post-mortem psychiatric evaluation by Sigmund Karterud and Ingar Sletten Kolloen concluded that Hamsun had an unspecified personality disorder but was legally sane.

[3] In 1966 he wrote the book Den gylne regel og andre humanistiske moralnormer (The Golden Rule and Other Humanistic Morale Norms).