Objetive 1. - To compare the capacity of the PET and the sentinel node for the detection of axillary metastasis (MTS). 2. - To evaluate sensitivity, specificity, positive and negative predictive value of the PET/CT. Inclusion criteria-Breast cancer T1 T2 NO. Palpable nodes suspicious of involvement. Exclusion criteria-Ductal carcinoma in situ, inflammatory carcinoma, recent biopsy of the breast, pregnancy or lactation, diabetes. Method 18-FDG PET/CT was made 15 days before the surgery. The sentinel node biopsy took place with the combined method (radioisotopes and patent blue) Material They were evaluated 54 patients. Age average: 59 years (37-79). T1: 36 patients (67%); T2: 18 patients (33%) Tumor size between 5 and 40 mm (average 19) clinically negative axilla 46 (85%); doubtful axilla 8 (15%). Stage I: 33 patients (61%); stage II: 21 patients (39%). Results Forty four (81%) of the tumors were invasive ductal carcinoma: 7 (13%) invasive lobular carcinoma; and 3 (6%) ductolobular. Fifteen patients had MTS in the SN (28%), of these 3 were micrometastasis. In 2 patients the PET/CT changed the stage since it was positive for bony and pulmonary MTS. Results PET in breast: Eleven negatives (FN: 20%). Four were invasive lobular carcinomas; 4 were T1a. Sensitivity 80%. VPP = 100%. False negative: 20%Result PET in axila: In 45 patients the PET was negative (83%). Six of them presented MTS in the sentinel node; 3 of them were micrometastasis. False negative: 40%. Sensitivity: 60%.VPN 87%. In 9 patients the PET were positive (17%) and the sentinel node was positive in all the cases. Specifity: 100%. VPP = 100%. In 8 patients the axilla was doubtful, of these only in one patient PET and the SN was positive. Conclusion 1.-PET/CT does not contribute advantages for the axillary staging in the initial stage because it does not manage to identify smaller MTS of 5 mm. 2.- A negative PET/CT does not replace the sentinel node biopsy. 3.- A positive PET/CT would indicate the necessity of making an axillary dissection still in clinically negative axilla. 4- In clinically doubtful axilla was coincidence between PET/CT and the SN as much in the negatives as in the positives. Key words: Occult axillary metastases. Positron emission tomography. Sentinel node biopsy. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P1-01-28.