We examine the education gradient in diabetes, hypertension, and high cholesterol. We take into account diagnosed as well as undiagnosed cases and use methods accounting for the possibility of unmeasured factors that are correlated with education and drive both the likelihood of having illness and the propensity to be diagnosed. Data come from the National Health and Nutrition Examination Survey 1999-2012. The education gradient in chronic disease varies by whether self-reported or objective disease measures are used. Education is negatively associated with having undiagnosed disease in some cases, but findings vary by how we define undiagnosed disease.