['To assess whether outcome in advanced breast cancer patients is related to metabolic response to endocrine therapy determined by FDG---PET.', "We retrospectively identified 21 consecutive breast cancer patients receiving endocrine therapy for metastatic disease (number of previous therapies 3.6 ±3.5; mean ±SD). All patients had been evaluated with at least 2 FDG---PET's. The first scan was performed by initiation of endocrine therapy. The second scan was performed after a mean of 3.8 ±1.14 months (m). Seventy---two FDG---avid lesions were identified and followed. The mean change in SUVmax (ΔSUVmax) was calculated per patient.", 'ΔSUVmax dichotomized using the group median as cut---off (8.6%) was predictive of progression free survival (PFS). The median PFS for the response---group (n=10, median ΔSUVmax ---20.9%) was 10.1 m. The median PFS for the progressive disease---group (n = 11, median ΔSUVmax 40.6%) was 6.7 m (log---rank testing p = 0.033).'] Does stable metabolic disease on FDG-PET provide information on response to endocrine therapy for breast cancer?