Wolfe's criteria were assigned to mammograms of 202 women without breast cancer. Parity decreased the frequency of P2 patterns but not DY. P1 and N1 patterns apparently increased at the expense of P2 patterns. For every birth, the probability that a P2 pattern changed to P1 or N1 was roughly 7 or 8%. This effect was not limited to the first pregnancy, but also held for additional pregnancies. Low body weight was associated with dysplasia and prominent duct patterns. Reported declines of radiographic density with increasing age and/or menopause were confirmed. Ethnic group was unrelated to parenchymal pattern.