We used the population-based tumor registry of Kaiser Permanente in the United States (Portland, OR) to analyze breast cancer incidence from 1960 to 1985. Overall, incidence rose 45% during this period. The largest increases occurred in women 60 years of age or older (74%) and in those 45-59 (36%). The rate in women aged 20-44 has remained essentially unchanged. Localized and regional disease showed similar increases. Review of medical records revealed that only a small portion of this increase was likely to result from increased screening activities. From the increased availability of receptor assays in a large proportion of cases since the mid-1970s, we observed that incidence of estrogen receptor-negative cancers rose 22%-27% between the mid-1970s and the mid-1980s. In contrast, incidence of estrogen receptor-positive tumors increased an average of 131% in the same period, perhaps implicating hormonal factors in the rising incidence of breast cancer.