Abstract While historians of homosexuality in the post-Stalin era have mainly focused on developments such as discussions about the decriminalization of homosexuality and the establishment of sexology, our understanding of conservative perspectives on homosexuality remains limited. This article introduces and analyses a previously classified medical doctoral thesis (habilitation) on homosexuality prepared and defended in the 1970s. This is currently the only accessible in-depth scholarly investigation of male homosexuality in this period. The article seeks to contribute to the thriving field of the history of homosexuality under late socialism by shedding some light on the rationale for continuing to criminalize consensual homosexual activity, from the perspective of a contemporary forensic expert. Secondly, the thesis, which was based on the criminal files of those convicted of consensual homosexuality in Leningrad (1961–1968) – a total of 107 men were convicted – provides additional data on the previously rather obscure realm of interaction between homosexuals and law enforcement agencies.