Using 1,973 breast tumours from women aged 40-69 participating in the Swedish two-county trial of mammographic screening for breast cancer, we examined the effect of histological type on prognosis and sojourn time (the duration of the preclinical screen-detectable phase) by age. The hypothesis of dedifferentiation, according to which a cancer of mixed malignancy grade drifts towards grade 3 as the more poorly differentiated part of the tumour grows faster than the well-differentiated part, was also assessed. Ductal carcinoma in situ, invasive ductal carcinoma of grade 1, mucinous carcinoma and tubular carcinoma were all associated with good survival. Ductal carcinoma of grade 3 was associated with poor survival. Ductal carcinoma of grade 2, lobular and medullary carcinoma were associated with intermediate survival. These patterns were much the same in women aged 40-49 as in women aged 50-69. In women aged 40-49, sojourn time was estimated at about 2 years regardless of histological type. For women aged 50-69, there was a marked association of sojourn time with histological type, the shortest sojourn time being observed for lobular (2 years) and medullary (1.2 years) carcinoma, and the longest for ductal carcinoma grade 1 (7.7 years) and tubular carcinoma (7.1 years). There was strong evidence of a potential to dedifferentiation. A mover-stayer mixture of Markov chain models estimated that, in women aged 40-54, 91% of ductal tumours have the potential to dedifferentiate and, in women aged 55-69, 38% of ductal tumours have such a potential.