The S phase fraction of breast biopsies of 110 patients determined in vitro using H-3-thymidine. In fibrocystic lesions the H-3-thymidine labeling index (H-3-TMI) increases parallel to the degree of dysplasia. In our opinion proliferating mastopathias are only those with a H-3-TMI of more than 1%. Invasive ductal mammary carcinomas with the histological structure demonstrate a strong variable proliferative activity histoautoradiographically as expression of their different growth dynamics.