CONTEXT: Family history is a risk factor for many common chronic diseases, yet it remains underutilized in primary care practice. BACKGROUND: Family Healthware is a self-administered, web-based tool that assesses familial risk for CHD; stroke; diabetes; and colorectal, breast, and ovarian cancer, and provides a personalized prevention plan based on familial risk. The Family Healthware Impact Trial evaluated the tool. DESIGN: In this cluster RCT, participants completed baseline and 6-month follow-up surveys. The intervention group used Family Healthware directly after the baseline survey. Controls used the tool after completing the follow-up survey. SETTING/PARTICIPANTS: Patients aged 35-65 years with no known diagnosis of these six diseases were enrolled from 41 primary care practices. MAIN OUTCOME MEASURES: The prevalence of family-history-based risk for coronary heart disease (CHD); stroke; diabetes; and colorectal, breast, and ovarian cancer was determined in a primary care population. RESULTS: From 2005 to 2007, 3786 participants enrolled. Data analysis was undertaken from September 2007 to March 2008. Participants had a mean age of 50.6 years and were primarily white (91%) women (70%). Of the 3585 participants who completed the risk assessment tool, 82% had a strong or moderate familial risk for at least one of the diseases: CHD (strong=33%, moderate=26%); stroke (strong=15%, moderate=34%); diabetes (strong=11%, moderate=26%); colorectal cancer (strong=3%, moderate=11%); breast cancer (strong=10%, moderate=12%); and ovarian cancer (strong=4%, moderate=6%). Women had a significantly (p<0.04) higher familial risk than men for all diseases except colorectal and ovarian cancer. Overweight participants were significantly (p<or=0.02) more likely to have a strong family history for CHD, stroke, and diabetes. Older participants were significantly (p<or=0.02) more likely to report a strong family history for CHD and stroke as well as colorectal and breast cancer. CONCLUSIONS: This self-administered, online tool delineated a substantial burden of family-history-based risk for these chronic diseases in an adult, primary care population. TRIAL REGISTRATION: NCT00164658.