OBJECTIVE: To determine the relationship between argyrophilic nucleolar organizer regions (AgNORs), ploidy and proliferative activity in breast carcinomas. STUDY DESIGN: AgNOR staining was performed in 25 cases of infiltrating ductal carcinoma of the breast, and several features were quantified by image analysis: nuclear area, mean value of the number of AgNORs (M_N_Ag), area of AgNORs (A_Ag), area of AgNOR per nucleus (A_Ag/N), A_Ag variance (A_Ag_V) and coefficient of variation of AgNOR area per nucleus (CV_A_Ag/N). These findings were correlated with ploidy and proliferative activity obtained by flow cytometry, also quantifying the latter with proliferating cell nuclear antigen (PCNA). Comparison of AgNORs with size of the tumor and lymph node status was also made RESULTS: We observed a good correlation between M_N_Ag and A_Ag with ploidy (P < .001), M_N_Ag and A_Ag with S-phase fraction (SPF) (P < .001), A_Ag with S+G2M (P < .02) and PCNA with M_N_Ag (P < .05). The mean value of A_Ag allowed division of the cases into two groups: diploid (D) and near diploid (ND) with low SPF (A_Ag < or = 2.50 microns2), and D and ND with high SPF and aneuploidy (A) (A_Ag > 2.50 microns2) (P < .05). There was a statistically significant correlation between M_N_Ag and A_Ag and between tumor size and lymph node metastases (P < .05). CONCLUSION: AgNORs correlate with various cell cycle and clinicopathologic features and will be used eventually as prognostic markers in breast cancer.