One hundred and sixty-three breast carcinomas occurring in women aged between 26 and 44 years were examined for pathological features, oestrogen and progesterone receptor status, proliferation as determined by Ki-67 labelling and the presence of c-erbB-2 and p53 protein, and were compared with a control group of carcinomas from women in the 50-67 years age group. Carcinomas occurring in women aged under 35 years had a significantly high incidence of being poorly differentiated and of having high proliferation rates. This group also had a significantly high incidence of p53 protein staining. Carcinomas in the under 30 years age group had a lower incidence of oestrogen and progesterone receptor positivity. No differences were found in c-erbB-2-positive staining between the groups. Infiltrating lobular carcinomas were only identified in women aged 40 years and over. There was a higher incidence of a family history in the 35-44 years age group (18%) than in the under 35 years age group (11%). Breast carcinomas occurring in women aged under 35 years are more aggressive. An important finding is the high incidence of p53 positivity, which may indicate genetic instability.