Background: Infiltration of non-haematopoietic organs by small lymphocytic lymphoma/chronic lymphocytic leukaemia (SLL/CLL) is not unusual in late-stage disease and thus quite frequently encountered in post-mortem examinations. However, primary manifestation of SLL/CLL in the prostate is rarely diagnosed. Patients and Methods: We report two cases of primary prostatic SLL/CLL, in one case in combination with prostate carcinoma, and discuss diagnostic pitfalls, pathophysiological mechanisms and therapeutic management, together with an overview of the literature. Conclusions: Lymphocytic infiltration of the prostate associated with obstructive symptoms is rare but can already occur in very early disease. Microscopically, SLL/CLL infiltration can be distinguished from chronic prostatitis by its pattern of infiltration and by immunohistochemistry. As the incidence of both SLL/CLL and prostatic carcinoma increases with age, composite tumours might occur more often in the future.