Two hundred ninety-one normal postmenopausal women without hormone replacement therapy were studied for their blood levels of CA125. The levels ranged from 0.1 to 31.8 U/ml (mean+/-SEM 5.5+/-0.3 U/ml). Twenty patients with endometrial adenocarcinoma who had preoperative CA125 levels below 50 U/ml were also studied. The levels of CA125 in the latter group ranged from 2.1 to 43.0 U/ml (17.2+/-2.3 U/ml) and the difference of the mean value of the two groups was statistically significant (P < 0.001). The CA125 levels in normal postmenopausal women have a weak negative correlation with their age (CA125 = -0.025 x age + 6.9, P < 0.001), whereas in cancer patients, a positive correlation was seen (CA125 = 0.6 x age + 11.5, P < 0.05). Patients with endometrial cancer showed a decrease in their CA125 levels after a hysterectomy and salpingo-oophorectomy (6.5+/-1.2 U/ml, P < 0.0001). These results confirmed that the normal range of the CA125 of menopausal women is much lower than that of cycling women and each laboratory should establish its own normal range for the population. Also, it was suggested that CA125 levels which apparently fall within the normal range but are high, however, for the respective age, may indicate that an extensive search for an endometrial malignancy is necessary.