Sabina Spielrein (1885–1942), Russian-Jewish pioneer of psychoanalysis, was highly esteemed by Sigmund Freud, and later by young left-wing analysts like Otto Fenichel, for being a creative thinker with a talent for stimulating new questions and original research (notably in ego psychology, child analysis, linguistics and neuropsychology). When she returned to her home country in 1923, however, her traces largely disappear. For this reason, the thirteen-page handwritten letter, which Spielrein wrote to Max Eitingon on 24 August 1927, in the run up to the tenth International Psychoanalytic Congress, is a particularly welcome discovery. She reports on professional and private matters, and above all we learn for the first time something about Spielrein's position in the disputes over the relation between psychoanalysis and Marxism in the Soviet Union. Spielrein's spirited engagement in the increasingly acrimonious debate over ‘Freudism’ and Marx-influenced behavioural science is sketched in the context of the development of Russian psychoanalysis, from its brief flowering under Trotsky's protection to its crushing under Stalin.