After initial interviews with 20,291 adults in the National Institute of Mental Health Epidemiologic Catchment Area Program, we estimated prospective 1-year prevalence and service use rates of mental and addictive disorders in the US population. An annual prevalence rate of 28.1% was found for these disorders, composed of a 1-month point prevalence of 15.7% (at wave 1) and a 1-year incidence of new or recurrent disorders identified in 12.3% of the population at wave 2. During the 1-year follow-up period, 6.6% of the total sample developed one or more new disorders after being assessed as having no previous lifetime diagnosis at wave 1. An additional 5.7% of the population, with a history of some previous disorder at wave 1, had an acute relapse or suffered from a new disorder in 1 year. Irrespective of diagnosis, 14.7% of the US population in 1 year reported use of services in one or more component sectors of the de facto US mental and addictive service system. With some overlap between sectors, specialists in mental and addictive disorders provided treatment to 5.9% of the US population, 6.4% sought such services from general medical physicians, 3.0% sought these services from other human service professionals, and 4.1% turned to the voluntary support sector for such care. Of those persons with any disorder, only 28.5% (8.0 per 100 population) sought mental health/addictive services. Persons with specific disorders varied in the proportion who used services, from a high of more than 60% for somatization, schizophrenia, and bipolar disorders to a low of less than 25% for addictive disorders and severe cognitive impairment. Applications of these descriptive data to US health care system reform options are considered in the context of other variables that will determine national health policy.