Since 1981, the Centers for Disease Control has collaborated with State health departments and the District of Columbia to conduct random digit-dialed telephone surveys of adults concerning their health practices and behaviors. This State-based surveillance system, which yields data needed in planning, initiating, and supporting health promotion and disease prevention programs, is described in this paper. Standard methods and questionnaires were used to assess the prevalence of personal health practices and behaviors related to the leading causes of death, including seatbelt use, high blood pressure control, physical activity, weight control, cigarette smoking, alcohol use, drinking and driving, and preventive health practices. Between 1981 and 1983, 29 States (includes the District of Columbia) conducted one-time telephone surveys. Beginning in 1984, most States began collecting data continuously throughout the year, completing approximately 100 interviews per month (range 50-250), with an average of 1,200 completed interviews per year (range 600-3,000). The raw data were weighted to the age, race, and sex distribution for each State from the 1980 census data. This weighting accounts for the underrepresentation of men, whites, and younger persons (18-24 years) in the telephone surveys and, for many health practices, provides prevalence estimates comparable with estimates obtained from household surveys. Nearly all (86 percent) of the States distributed selected survey results to other State agencies, local health departments, voluntary organizations, hospitals, universities, State legislators, and the press. The majority (60 percent) of States used information from the surveys to set State health objectives, prepare State health planning documents, and plan a variety of programs concerning antismoking, the prevention of chronic diseases, and health promotion. Further, nearly two-thirds (65 percent) used results to support legislation, primarily related to the use of tobacco and seatbelts. Most of the States (84 percent) reported that alternative sources for such data (prevalence of behavioral risk factors) were unavailable. Currently in 1988, over 40 State health departments are conducting telephone surveys as part of the Behavioral Risk Factor Surveillance System. This system has proved to be (a) flexible--it provides data on emerging public health problems, such as smokeless tobacco use and AIDS, (b) timely--it provides results within a few months after the data are collected, and (c) affordable--it operates at a fraction of the cost of comparable statewide in-person surveys. The system enables State public health agencies to continue to plan,initiate, and guide statewide health promotion and disease prevention programs and monitor their progress over time.