The diagnostic efficacy of fine needle aspiration cytology of breast masses and the causes for unsatisfactory specimens in private practices were investigated in Hong Kong. All specimens that were submitted to the histopathology unit at the Canossa Hospital between 1 January 1996 and 30 April 1997 formed the basis of this report. A total of 1533 specimens were received from 1447 patients; 274 (17.8%) cases were unsatisfactory for assessment, 1080 (70.4%) were benign, 51 (3.3%) atypical, 19 (1.2%) suspicious, and 67 (4.4%) malignant. The specimens were submitted by 105 doctors, who each performed between 1 and 561 smears. The proportion of unsatisfactory samples was high for doctors who performed an occasional fine needle aspiration (48%; overall mean, 25%). Histological correlation was available in 165 cases. The overall sensitivity was 79%, specificity 98%, positive predictive value (including the ‘suspicious’ category) 92%, and negative predictive value 94%. Two false positive cases that were reported as suspicious were found to be fibroadenomata following subsequent excision biopsy. No adverse clinical outcomes were recorded for the false positive cases. There were six false negative cases (reported as a cyst in one case, benign in two cases, and atypical in three cases). The results compared favourably with published data and affirmed the effectiveness of the test in private practice.