We report a case of seventy-four year-old male with advanced urinary tract obstruction symptoms and a high serum level of prostate specific antigen. In the microscopic examination of the transrectal ultrasound guided prostate needle biopsy specimens, a diffuse infiltration composed of small lymphocytes which were strongly positive for CD20 and CD5 (30%) and negative for CD3, CD10, CD43 and Tdt. After his referral to the hematology clinics for further investigation to verify a possible low grade lymphoproliferative neoplastic disorder, an absolute lymphocytosis in the peripheral blood and generalized lymphadenopathies were detected. In the bone marrow trephine biopsy, we observed a diffuse infiltration of small lymphocytic lymphoid cells having the same morphological and immunohistochemical features with the cells detected in prostate biopsies. With these findings, we made the diagnosis of small lymphocytic lymphoma/chronic lymphocytic leukemia and its prostatic involvement.