In a prospective adjuvant trial including patients with primary operable breast cancer, invasive carcinomas of ductal type were subjected to histological malignancy grading. The parameters investigated were tubule formation, number of mitoses and cell pleomorphism. A Cox regression model for survival data was applied to evaluate the impact of the histological parameters on prognosis in 1809 patients with low-risk carcinomas. Cell pleomorphism proved superior to the other histologic tumor characteristics. It was found that low-risk invasive ductal carcinomas with severe cell pleomorphism had an excess recurrence intensity of 209% relative to carcinomas with no pleomorphism. It is therefore suggested that polymorphous invasive ductal breast carcinomas, other things being equal, should be regarded as high-risk tumors in future clinical trials. Finally it was found that the tripartite malignancy grading 1, 2 and 3 characterizing each of the histological parameters was not equidistant. Consequently, the traditional tripartite histologic scoring needs reconsideration.