The DNA content of 85 ductal breast cancers of different histological grades was evaluated using static cytometry and correlated with immunocytochemical expression of p53 protein in tumour cells in cytological material. A statistically significant difference was observed between p53 protein expression and grade of malignancy (P < 0.001). The percentage of euploid tumours significantly decreased from grade I through grade II to grade III tumours (P < 0.001). Clonal DNA heterogeneity was observed in 26.6% of cases analysed and was correlated with p53 protein expression (P < 0.001). These changes probably reflect genomic alterations which may affect potential malignancy of breast cancer.