CONTEXT: Knowledge of family cancer history is important for assessing cancer risk and guiding screening recommendations. OBJECTIVE: To quantify how often throughout adulthood clinically significant changes occur in cancer family history that would result in recommendations for earlier or intense screening. DESIGN AND SETTING: Descriptive study examining baseline and follow-up family history data from participants in the Cancer Genetics Network (CGN), a US national population-based cancer registry, between 1999 and 2009. PARTICIPANTS: Adults with a personal history, family history, or both of cancer enrolled in the CGN through population-based cancer registries. Retrospective colorectal, breast, and prostate cancer screening-specific analyses included 9861, 2547, and 1817 participants, respectively; prospective analyses included 1533, 617, and 163 participants, respectively. Median follow-up was 8 years (range, 0-11 years). Screening-specific analyses excluded participants with the cancer of interest. MAIN OUTCOME MEASURES: Percentage of individuals with clinically significant family histories and rate of change over 2 periods: (1) retrospectively, from birth until CGN enrollment and (2) prospectively, from enrollment to last follow-up. RESULTS: Retrospective analysis revealed that the percentages of participants who met criteria for high-risk screening based on family history at ages 30 and 50 years, respectively, were as follows: for colorectal cancer, 2.1% (95% confidence interval [CI], 1.8%-2.4%) and 7.1% (95% CI, 6.5%-7.6%); for breast cancer, 7.2% (95% CI, 6.1%-8.4%) and 11.4% (95% CI, 10.0%-12.8%); and for prostate cancer, 0.9% (95% CI, 0.5%-1.4%) and 2.0% (95% CI, 1.4%-2.7%). In prospective analysis, the numbers of participants who newly met criteria for high-risk screening based on family history per 100 persons followed up for 20 years were 2 (95% CI, 0-7) for colorectal cancer, 6 (95% CI, 2-13) for breast cancer, and 8 (95% CI, 3-16) for prostate cancer. The rate of change in cancer family history was similar for colorectal and breast cancer between the 2 analyses. CONCLUSION: Clinically relevant family history of colorectal, breast, and prostate cancer that would result in recommendations for earlier or intense cancer screening increases between ages 30 and 50 years, although the absolute rate is low for prostate cancer.