As the US population ages, the increasing prevalence of chronic disease and complex medical conditions will have profound implications for the future health care system. We projected future prevalence of selected diseases and health risk factors to model future demand for health care services for each person in a representative sample of the current and projected future population. Based on changing demographic characteristics and expanded medical coverage under the Affordable Care Act, we project that the demand for adult primary care services will grow by approximately 14 percent between 2013 and 2025. Vascular surgery has the highest projected demand growth (31 percent), followed by cardiology (20 percent) and neurological surgery, radiology, and general surgery (each 18 percent). Market indicators such as long wait times to obtain appointments suggest that the current supply of many specialists throughout the United States is inadequate to meet the current demand. Failure to train sufficient numbers and the correct mix of specialists could exacerbate already long wait times for appointments, reduce access to care for some of the nation's most vulnerable patients, and reduce patients' quality of life.