The appearance of hematopoietic marrow in magnetic resonance (MR) imaging is variable and differences between normal and pathologic marrow may be subtle. To aid in the evaluation of this problem, we reviewed 82 consecutive pelvic MR examinations in men with no evidence of osseous metastases. Images were evaluated with regard to the overall fraction of residual hematopoietic marrow present and the characteristics of this marrow. The patient population in our study was older (mean age 66 years) than the patient populations in previous papers documenting normal marrow patterns. The overall amount of hematopoietic marrow present was less in this older patient population, with 80% of patients having less then 40% residual hematopoietic marrow. A consistent pattern of morphologic change was noted as hematopoietic marrow converted to fatty marrow with increasing age. Initially, hematopoietic marrow tended to appear diffuse, heterogeneous, and with poorly defined margins on MR imaging. As conversion to fatty marrow continued, hematopoietic marrow became more focal and sharply defined, usually in the form of islands of residual hematopoietic marrow. Periarticular hematopoietic marrow predominated in the sacroiliac region (72% of patients) with little residual hematopoietic marrow noted in the symphysis pubis (5%) and hip joints (30%). Hematopoietic marrow persisted longer in juxtacortical locations (87%), was always symmetric (100%), remained less intense than fat on T2-weighted images (100%), and usually had a central focus of fat (98%). These morphologic criteria may be of value in establishing the MR appearance and patterns of marrow in the pelvis, and in the recognition and confident diagnosis of foci of hematopoietic marrow.