Genomic DNA was extracted from aspiration biopsy specimens taken from 15 suspected cases of breast cancer, including 7 known cases of breast cancer, and the p53 gene was studied for evidence of mutation by using a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. In 5 of the 15 cases (33%), p53 gene mutation was identified and these tumors were subsequently histologically diagnosed as malignant. Further, DNA flow cytometry of the 15 tumors demonstrated that 6 (40%) were aneuploid and malignant, whereas 9 (60%) were diploid and benign. It was also found that the tumor cells in 5 aspirated cases that showed p53 gene mutations were all aneuploid, the p53 protein expression was positive, and the tumors were proved to be histologically malignant. It was thus concluded that the detection of p53 gene mutation by PCR-SSCP analysis of aspirated biopsy specimens from suspected breast cancers is a helpful method for achieving a more accurate diagnosis.