The internal calcar septum is a ridge of cortical bone protruding from the inner cortical wall of the proximal femur into the medullary canal. It extends from the lesser trochanter into the femoral neck and narrows the femoral cavity in its dorsal third. This region is essential for THR stability, but the degree of contact between the septum and standard THR implants has never been studied. We obtained CT scans of 50 arthrotic hip joints from patients requiring THR. Virtual stems (50 straight/wedge-shaped and 50 anatomic stems) were placed in CT images of the femora using a PC-based preoperative planning unit. The dimensions of the septum, degree and location of contact between the septum and implants were recorded. A septum of cortical density was seen in 49/50 CT scans. It was 11 (2.9) mm long (medial-lateral), 3.5 (0.7) mm wide and 32 (10) mm high (caudal-cranial, mean (SD)). 94/100 implanted virtual stems showed direct contact with the septum. 31 straight stems and 5 anatomical stems were supported by the septum along their dorsal side. The internal calcar septum can be consistently seen on CT scans of patients needing THR and it probably contributes to THR stability.