This study examined age differences in the quality of self-report data in patients with chronic disease conditions (hypertension, diabetes, heart disease, depression). Data are from 2,304 patients in three health care systems in Los Angeles, Chicago, and Boston. Results support the idea that self-report health data can be gathered from older and younger patients without significant decrements in data quality. Specifically, results showed: (1) small decreases in the reliability of multi-item measures with age, primarily occurring in balanced scales; (2) little evidence of differences among age groups in response set or the tendency to respond "don't know" or "uncertain," although older patients had a greater tendency to respond in a socially desirable manner; (3) higher item nonresponse in older patients; (4) little variation in item nonresponse by type of question or question placement; (5) generally high panel retention in all age groups, supporting the value of repeated follow-up; and (6) similar known-groups validity across age groups.