Specific atypical histological patterns of epithelial hyperplasia (AH) indicate a medically relevant risk of breast cancer development in 5-10% of women with otherwise benign biopsies. This risk is about four times that of similar women, i.e., of the same age and at risk for the same length of time. These relative risks are not stable with time and fall 10-15 years after detection. Absolute risk for invasive breast cancer after AH is about 10% in 10-15 years after biopsy and is most certain for perimenopausal women. Proliferative disease without atypia predicts only a slight elevation of risk with a relative risk (RR) of 1.5 to 2 times that of the general population. There is such a strong interaction between family history and AH that it is relevant to consider women with atypical hyperplasia who have a positive family history (FH) of breast cancer separately from those who do not. The absolute risk of breast cancer development in women with AH without a FH was 8% in 10 years (RR about 4), whereas those with a positive family history experienced a risk of about 20% at 15 years (RR of about 10). This interaction of AH and FH has also been observed in other recent studies. Low replacement doses of conjugated estrogen after menopause do not further elevate risk beyond that identified by histology.(ABSTRACT TRUNCATED AT 250 WORDS)