Whereas efforts encouraging women to obtain initial mammograms are laudable, the importance of returning for subsequent routine mammograms cannot be minimized. The purpose of this study was to measure the timing, patterns, and characteristics of repeat screening mammography over time in a defined population of health maintenance organization members for whom mammography was a fully covered benefit. We identified all women ages 50-74 years who were enrolled in a southeastern Michigan health maintenance organization, assigned to a large medical group, and received at least one screening mammogram with a normal result between January 1, 1989 and December 31, 1996. Using administrative and radiology data, we calculated the proportion of women who received a subsequent mammogram within 2 years and the time to subsequent screening, both overall and stratified by demographic characteristics. We also examined screening patterns over a 5-year period. Of the 8749 women included in this study, 66.0% [95% confidence interval (CI), 65.0-67.0%] were subsequently screened within 2 years. We found slightly higher rates among Caucasians and married women. The proportion of women who received repeat mammography increased with estimated household income [9.5% difference between the highest and lowest categories (95% CI, 6.5-12.5%)]. The median time to subsequent screening was 17.7 months, and the probability of repeat screening was higher for women whose initial mammogram was between January 1992 and December 1994 compared to those receiving an initial mammogram between January 1989 and December 1991 (9.6% difference; 95% CI, 7.5-11.7%). Repeat mammography has improved over time; however, socioeconomic status could contribute to longer-than-intended intervals between screening when translated into real-world clinical practice. In a setting where most physicians recommended annual screening, we found that the median time to subsequent screening was delayed by 6 months. If annual mammography is the goal, recommendations should be made with the understanding of how the timing of repeat screening occurs in clinical practice.