All cases of ductal carcinoma in situ (DCIS) diagnosed from 1987 to 1991 in the Southern Health Care Region of Sweden, and operated upon with breast conserving treatment (BCT) with (n=66) or without (n=121) postoperative radiation (RT) were clinically followed, morphologically re-evaluated and analysed for cell biological factors (immunohistochemical assays or DNA flow cytometry). Median age at diagnosis was 58 years (range 29--83 years) and median follow-up was 62 months. Oestrogen (ER)- and progesterone receptor (PR)-negativity, c-erbB-2 overexpression, low bcl-2 expression, p53 accumulation, DNA non-diploidy and high Ki67, were strongly associated with high grade DCIS, and comedo-type necrosis. In contrast, significant associations to growth pattern (not diffuse versus diffuse) were seen only for c-erbB-2 and PgR. There was also a strong relationship between the cell biological factors, and a summary cell biological index based on principal component analysis was introduced (CBI-7). In the group that had not received postoperative RT, 31 ipsilateral local recurrences occurred (13 invasive, 18 DCIS). Ipsilateral recurrence-free interval (IL-RFI) was in univariate analyses significantly, or almost significantly, shorter for patients showing p53 accumulation, high Ki67 or low bcl-2, compared with patients with normal p53, low Ki67 and high bcl-2. The prognostic importance of the remaining cell biological factors was less pronounced. On the other hand, the index CBI-7, was a strong predictor for recurrence.