Needle track seeding has been recognized as a possible, albeit rare, complication of breast core needle biopsy. The purpose of this study was to assess the risk of needle tract seeding of breast cancer from cytological results derived from core needle wash material. The study included biopsies of 207 breast cancers performed using ultrasonographically guided 18-gauge core needles. Each core needle without exposed sample notch was washed in saline solution immediately after removing the needles. Cytology derived from core wash material was performed by saline solution lavage of the fragments using a cytocentrifuge. The cytological diagnoses were divided into five categories: benign, atypical/indeterminate, suspicious/probably malignant, malignant and unsatisfactory. Atypical/indeterminate, suspicious/probably malignant and malignant categories were considered to represent positive cases of needle track seeding of breast cancer, whereas benign and unsatisfactory categories were counted as negative cases. Cytological diagnoses of the 207 lesions were as follows: 26 lesions (12%) were benign, 18 lesions (9%) were atypical/indeterminate, 37 lesions (18%) were suspicious/probably malignant, 79 lesions (38%) were malignant, and 47 lesions (23%) were unsatisfactory. The incidence of positive cases of cytology derived from core wash material was 65% (134/207). The 25% frequency of positive cases of invasive lobular carcinoma was significantly lower than the frequencies of DCIS (74%) and invasive ductal carcinoma (69%) (P = 0.001 and P < 0.01). The frequency of positive cases in the multiple passes group was 75%, which was slightly, although not significantly, higher than the 66% frequency in the single pass group (P = 0.3). In conclusion, the incidence of positive cases of cytology derived from ultrasonographically guided breast core needles' wash material was 65%. The clinical significance is debatable; however, there may be a theoretical risk of local recurrence if the tract is not excised or radiotherapy not given.