The aim of this pilot study was to determine the postoperative blood perfusion (BF PET ) and perfusion heterogeneity (BF PET HG) in free microvascular breast reconstruction flap zones with positron emission tomography (PET). Regional BF PET and BF PET HG of the adipose tissue in medial, central, and lateral parts of 13 free flaps were assessed on the first postoperative morning with PET using oxygen-15-labeled water ([ 15 O]H 2 O) in 12 patients undergoing breast reconstruction with a deep inferior epigastric perforator (DIEP) or a transverse rectus abdominis muscle (TRAM) flap. The mean BF PET values did not differ between DIEP and TRAM flaps (P 5 0.791). The mean BF PET values were higher in zone III compared with zone I (P 5 0.024). During follow-up, fat necrosis was identified in three patients in the medial part (zone II) of the flap. However, the adipose tissue BF PET assessed on the first postoperative day from all zones of the flap using PET with radiowater was normal. The BF PET HG was higher in the control side (i.e., in the healthy breast tissue) compared with the flap (P 5 0.042). The BF PET HG was lower in zone III than in zone I (P 5 0.03) and in zone II (P < 0.001). In this pilot study, PET was used for the first time for studying the adipose tissue perfusion in different zones in free flaps in a clinical setup, finding that the mean BF PET values did not differ between DIEP and TRAM flaps, and that zone II was sometimes not as well perfused as zone III supporting revisited zone division. V