Mammograms of 15,110 women, representing a random selection of all 45-69-year-old women living in Malmö and taking part in screening for carcinoma of the breast, were analyzed using Wolfe's criteria for variations in the parenchymal pattern with (a) age at examination and (b) age at the birth of her first child. P2 and DY, which are considered high-risk patterns, increased in frequency with advancing age at first birth, being highest in nulliparous women, but decreased with age at examination.