This study was undertaken to determine the frequency and distribution of dense mammograms. Factors that may affect parenchymal density of breasts among the diverse age groups of women who undergo mammography were also studied.

Mammograms of 1353 women between 25 and 79 years old who were grouped in 5-year age cohorts were reviewed. Breast density and the presence of benign calcifications were analyzed.

Parenchymal density on mammograms decreased progressively in the patient cohorts 25-29 years old through 75-79 years old (Spearman correlation, p < .01). In the cohort of 25- through 29-year-old patients, 38% had predominantly (> 50%) fatty breasts. In the cohort of 75- through 79-year-old patients, 76% had predominantly fatty breasts. Increased parenchymal density mammograms were more common in women who had smaller breasts, had had fewer than two pregnancies, and underwent hormone replacement (p < .01). Forty-nine percent of women 50-79 years old undergoing hormone replacement had predominantly dense breasts, a percentage similar to that (48%) of the patient cohort of women 40-44 years old. Prevalence of benign calcifications also increased with age, from 8% at ages 25-29 to 86% at ages 75-79 (p < .01).

In our study, a significant percentage (38%) of women who were 25-39 years old had predominantly fatty breast tissue that should not impede selective mammographic screening or diagnostic efforts in this age group.