635 Background: Fine-needle aspiration cytology (FNAC) has been used extensively in the diagnosis of breast lesions, but false-negative rates are a matter of concern. However, immediate onsite evaluation of breast lesions, combining FNAC results with clinical and radiological data allows to improve its diagnostic accuracy. The objective of this study was to evaluate the concordance between cytopathological and histological results in a large series of FNAC performed during the first 3-yr period of the dedicated one-stop clinic set up in our institution since 2004. METHODS: We reviewed data of all patients (pts) seen in the one stop clinic between May 2004 and March 2007 who had a lesion diagnosed by FNAC and verified by histology. For benign lesions, histological verification of the lesion was mandatory each time there was no perfect concordance between clinico-radiologic features and FNAC results (such as benign FNAC but BI-RAD 5). Pts characteristics, radiological findings, cytopathological and histological (by per-cutaneous biopsy or surgery) results were extracted from the hospital computerized prospectively registered medical records. RESULTS: A total of 1053 nodular breast lesions (mean size: 24+/-23 mm, BI-RAD ACR 1/2/3/4/5: 2/10/68/283/684) in 1015 pts (mean age: 59+/-13 years) were studied. FNAC was US-guided in 521 lesions (49.4%). FNAC classified lesions as malignant in 741 (70.4%), benign in 127 (12.0%), suspicious in 143 (13.6%), and unsatisfactory in 42 (4.0%) cases. Among definitive benign or malignant diagnosis, FNAC and histological results showed concordance in 840/868 (96.7%) cases (738 malignant and 102 benign cases). The numbers of false-negative and false-positive lesions were respectively 25/127 (19%) and 3/741 (0.4%). Among the 143 suspicious and 42 unsatisfactory specimens, 114 and 20 were malignant, respectively. CONCLUSIONS: FNAC performed in a one-stop clinic allows obtaining an immediate on-site diagnosis of breast cancer with a good concordance with histological results and low rate of unsatisfactory specimens. However, affirming benign lesion requires a multi-disciplinary team: whenever clinico-radiologic features and FNAC results are non concordant, histologic assessment is mandatory. No significant financial relationships to disclose.