In our study, the expression of Ki-67 antigen in phyllodes tumours of the breast was investigated in order to estimate its significance in assessing the grade of malignancy and in predicting the clinical behaviour of these tumours. Formalin-fixed and paraffin-embedded tissue specimens of 37 cases of phyllodes tumour of the breast (eight graded as malignant and 29 as benign according to histopathological criteria) and 20 breast fibroadenomas were stained by an immunoperoxidase technique, using the monoclonal antibody MIB1 directed against cell proliferation-associated Ki-67 antigen. A MIB1 index was calculated to express the percentage of MIB1 positive proliferating stromal cells. Our data indicate that there are statistically significant differences in MIB1 indices between histologically benign and malignant phyllodes tumours (alpha = 0.001), between benign phyllodes tumour and fibroadenoma (alpha = 0.01), and between malignant phyllodes tumour and fibroadenoma (alpha = 0.001). Our results also indicate a good correlation between the conventional grading of phyllodes tumours based on histological criteria and MIB1 indices. However, little predictive value of the MIB1 index in phyllodes tumours of the breast can be derived from our study.