Premorphological metabolic changes in human breast carcinogenesis. Malignant breast tissue is characterized by morphological and metabolic changes when compared with normal breast tissue. In this study, the cytochemical measurement of glucose-6-phosphate dehydrogenase (G6PD) activity was used to detect abnormal metabolism in breast tissue and to determine whether abnormal metabolic activity precedes morphological change during human breast carcinogenesis. Normal and benign breast tissue, morphologically normal tissue from cancer-containing breasts, and malignant breast tissue were studied. In malignant tissue, mean(s.e.m.) G6PD activity was significantly increased when compared with normal and benign tissue (9.69(2.3) versus 27.02(1.7) mean integrated extinction (MIE) x 100, P less than 0.01). G6PD activity was increased in morphologically normal tissue from cancer-containing breasts when compared with normal and benign breast tissue from breasts with no known cancer (27.02(1.7) versus 18.42(2.6) MIE x 100, P less than 0.05). These findings suggest that metabolic abnormalities precede morphological changes in breast carcinogenesis. Abnormal metabolism can be detected widely within a cancer-containing breast. The detection of such abnormality may prove helpful in identifying patients at high risk of developing breast cancer. 