Frozen section diagnosis (FSD) given in 4436 consecutive breast biopsies performed in 5 years in a single pathology laboratory were checked against the final pathological report. In 4284 cases (96.57%) there was no difference between the FSD and the definitive diagnosis. There were 74 (1.66%) false negative reports and no false positive diagnoses. The diagnosis was deferred to paraffin sections in 78 cases (1.75% of biopsies). The predictive value for positive results was 100% and for negative results 97.5%; the specificity was 100%, the sensitivity 94.6% and the accuracy 98.3%. Minimal breast cancer, in situ (CIS) especially, was the main source of false negative reports. In non minimal invasive cancers (NMIC) FSD was correct in 99.42%. In minimal invasive cancers (MIC) FSD was correct in 80.21%, false negatives and deferred diagnosis increased to 8.79% and 10.98%. In CIS false negatives increased to 76.82% and deferred diagnoses to 12.19%. The sensitivity of fine needle aspiration, performed before biopsy in a portion of the patients, was lower than FSD in NMIC (71.39% versus 99.21%) and in MIC (41.66% versus 80.55%), identical to FSD in CIS (7.40% versus 7.40%). The value of cytodiagnosis in addressing surgery is discussed.