For the first time ever, this study presents facts on the biography of Nikolaus Jensch (1913 - 1964), who worked in Wroclav, Leipzig, Strasbourg and Bremen, and on his psychiatric-genetic studies on homosexuality during the Nazi regime. Jensch tried, under methodologically rather doubtful presuppositions, to separate "genuine" from so-called "pseudo-homosexuals", who for him were receptive to psychotherapeutic and educational intervention. Ultimately he aimed to establish an empiric prediction as to the heredity of homosexuality, although even that would not exculpate homosexuals from prosecution. In a different study he analysed the curing effects of castration for homosexuality and other sexual deviances. Although acknowledging the mutilating character of castration and its due to high recrudescence rates rather poor therapeutic outcome, he never pleaded to refrain from this intervention.