Ornithine decarboxylase (ODC) activity in human breast cancer tissues was correlated with prolactinemia (Prl), estradiol and progesterone cytosol receptors (ER and PR), and histopathologic pattern. Ninety-two cases of breast cancer, six benign mammary disease, and three normal breast tissues were studied for ER, PR, and ODC. Prolactinemia was assessed in 59 cancer patients, 14 of whom showed hyper-Prl along with significantly higher ODC than in patients with normal-Prl [(20.01 +/- 6.33) 10(-2) vs (5.20 +/- 0.90) 10(-2) pmol CO2/micrograms protein/h; P less than 0.0125]. A direct correlation was found between Prl and ODC in postmenopausal women (n = 40). Prl was assayed in seven of 13 ER-PR breast cancer patients; a highly significant, direct correlation was found between Prl and ODC in this group (r = 0.934, P less than 0.0025). ODC did not correlate with ER or PR. Carcinomas with higher ODC (n = 17) had higher cellularity, lower histologic differentiation, and higher nuclear anaplasia than those in which ODC was not detectable (n = 13). In normal breast and five of six benign mammary disease tissues, ODC was not detectable. These findings suggest that ODC could be a reliable marker for prognosis.