Objective: To investigate whether ultrasound (US) guidance improves the sensitivity of fine-needle aspiration biopsy (FNAB) in patients with palpable breast tumors. Design: A retrospective study. Setting: A university hospital. Patients: A total of 107 patients with breast tumors (114 lesions) underwent US-guided FNAB and 138 patients (143 lesions) underwent FNAB guided by palpation (standard FNAB). Main Outcome Measures: The diagnoses obtained by FNAB were compared with the surgical findings and the diagnostic value of US-guided FNAB was evaluated. Results: The sensitivity of US-guided FNAB was 89.3%, the specificity was 82.9%, and the accuracy was 86.9%. In contrast, the sensitivity of standard FNAB was 61.1%, the specificity was 73.3%, and the accuracy was 65.0%. For breast tumors 2.0 cm or less in diameter, the sensitivity and accuracy of US-guided FNAB were significantly higher than those of standard FNAB. Conclusion: Ultrasound-guided FNAB can improve the preoperative diagnosis of breast cancer, especially in patients with tumors that are 2 cm or less in diameter.