Objective: Fine needle aspiration cytology is the first step in the diagnosing breast lesions. This study evaluated factors causing falsenegative and false-positive diagnoses when evaluating breast lesions using this technique.Material and Method: In this study, we retrospectively examined 511 breast diagnoses, based on Fine needle aspiration cytology specimens, made in the Medical School of Zonguldak Karaelmas University, Department of Pathology, between 2002 and 2009. Factors affecting the reliability of fine needle aspiration cytology were evaluated by comparing the cytological and biopsy diagnoses and using the clinical parameters in the diagnosis of breast lesions.Result: In our series, the sensitivity, specificity, accuracy, positive predictive value, and negative predictive value of fine needle aspiration cytology were 77%, 99%, 95%, 93%, and 95%, respectively. The falsenegative diagnosis rate was 4% and the false-positive diagnosis rate was 1%.Conclusion: Sampling errors and erroneous interpretation of cellular monomorphism are the most important reasons for false-negative diagnosis results in the evaluation of breast lesions with fine needle aspiration cytology. Increased cellularity and reactive cell atypia in benign proliferative breast lesions are the most frequent reasons for false-positive diagnosis.