Accurate presurgical assessment of tumor size is important for choosing appropriate treatment, especially with the increasing use of neoadjuvant and minimally invasive therapy. Breast sonography is increasingly used by breast surgeons as a part of their basic clinical evaluation. We undertook this study to compare clinical evaluation, mammography, and breast sonography for evaluating breast tumor size. A prospective analysis of 124 consecutive patients with palpable breast cancer was performed. Tumor masses belonging to T1 and small T2 were selectively selected. All women had clinical, mammographic, and sonographic assessment of tumor size. Measurements were compared to the pathologic tumor size of the surgical specimen. Both mammographic and sonographic measurements tend to underestimate tumor size, while clinical assessment tends to overestimate it. Ultrasound was significantly more accurate in determining tumor size. The maximal tumor diameter measured was within 2 mm of the pathologic tumor size in 45.2% of cases measured by breast ultrasound, 28.2% of cases measured by mammography, and 14.5% of cases measured clinically. These data suggest that ultrasound is more accurate than clinical breast examination and mammography in assessing breast cancer size. Ultrasound assessment should be used by surgeons as an accurate adjunct to clinical examination in outpatient breast clinics.