Focal apocrine metaplasia was studied in 293 whole human breasts by a subgross sampling technique with histologic confirmation. There were 186 breasts obtained from random autopsies. Another 107 breasts were obtained that were cancer-associated; these breasts either contained an invasive cancer or were situated contralateral to a cancer-containing breast. The observations support a lobular origin for most if not all apocrine metaplasia and demonstrate some correlation between the presence of apocrine metaplasia and coincident invasive breast cancer on either the contralateral or ipsilateral side. When extensive, apocrine metaplasia appears to be a useful phenotypic marker for tissue at a modestly increased risk for breast cancer. There were no breast carcinomas of the apocrine type in this series, supporting the belief that apocrine metaplastic epithelium has little intrinsic malignant potential. Finally, the data support the hypothesis that most breast cysts arise from apocrine metaplastic lobules.