Systematic surveillance was conducted of CHF among residents, 35-84 years old, of the Minneapolis-St.Paul metropolitan area (population 2.45 million, 1995 estimate) in two complementary domains: 1) hospitalization for CHF in two calendar years, five years apart (1995 and the year 2000); and 2) newly diagnosed CHF over a 9-year period (1993-2001) among members of a large Health Maintenance Organization (HMO). The magnitude and characteristics of hospitalized CHF were assessed in all 21 acute care hospitals of the metropolitan area. One-third of all discharges with ICD-9 CHF codes were sampled randomly (n about 5,000 per surveillance year), abstracted by trained nurses, and classified according to clinical criteria and the results of diagnostic tests. Newly diagnosed CHF was distinguished from recurrent episodes of decompensated CHF by thorough review of the hospital record. A complementary perspective on CHF, including the outpatient setting, was provided by a systematic study of members of HealthPartners, one of the largest HMOs in Minneapolis-St. Paul. Using the HealthPartners database, all newly diagnosed CHF cases between January 1, 1993 and December 31, 2001 (n about 1,900) were identified and the diagnosis validated. Beginning in January 1, 1999, newly diagnosed CHF cases were identified on an ongoing basis (n about 600), surveyed by mail, and followed for one-year from the original diagnosis. Surveillance of the HealthPartners population supplemented hospital surveillance and contributed data on incidence, prevalence, treatment, use of resources, and patient outcomes.