Fat necrosis in transverse rectus abdominis musculocutaneous (TRAM) flap is considered to be mainly affected by blood supply. This prospective study compares the incidence of fat necrosis between zones II and III in 400 consecutive patients who had undergone unipedicled TRAM flap breast reconstruction. Fifty-eight patients (14.5%) suffered from fat necrosis, and 7 had 2 separate nodules. Fifty-four occurred in zone II, 10 in zone III, and 1 in zone I. The incidence of fat necrosis in zone II was significantly higher than in zone III (P < 0.001). The weight of the mastectomy specimen and the relative amount of zone II tissue included in the flap had positive correlation with the incidence of fat necrosis. This result implies relatively poor perfusion of zone II compared with zone III.