Histopathological malignancy grading using the Bloom-Richardson classification of ductal cancers was performed for 248 invasive ductal breast cancers < or = 10 mm operated 1978-1985. There were significantly more grade 1 lesions in the prevalence screening round. Grade 3 was correlated with aneuploidy, higher S-phase (SPF), and more receptor negative tumours. There were also significantly more positive lymph nodes in grade 3 lesions, 18% compared to 5% and 12% respectively for grades 1 and 2 (p < 0.05). In life table analysis for survival, when the high risk group of grade 3 lesions was compared to the grade 1 and 2 lesions combined, five-year disease-free survival was 84.6% vs. 99.1% (p < 0.001). With good training and care from the pathologist, malignancy grading seems useful for prognostication of eventual recurrence and death. In tumours 10 mm or smaller only grade 3 lesions need to be included in follow-up systems and should probably have adjuvant treatment. Malignancy grading is especially good in small ductal breast cancers where grading can always be performed while other prognostic determinations are hampered by shortage of material. Lymph node positivity is also low in this group.