Objective: To examine the impact of a new chronic disease diagnosis on substance use (i.e., smoking and drinking), utilization of preventive medical procedures, and physical activity among middle-aged and older adults. Method: Individual-level data came from 1996 to 2010 waves of the U.S. Health and Retirement Study. Disease diagnosis was ascertained from self-reports of physician-diagnosed diseases. A case-control difference-in-differences approach estimated in logistic regression was applied to test study hypotheses. Results: After a diagnosis of chronic disease, participants decreased substance use and increased utilization of preventive medical procedures. Physical activity declined after a diagnosis of lung disease, cancer, and stroke. Discussion: Chronic disease diagnosis may be an important teachable moment that can motivate individuals to adopt multiple risk-reducing health behaviors. Future research needs to elucidate the mechanisms through which disease diagnosis affects behaviors and test the modifying effect of time since diagnosis on intervention effectiveness.