Many factors may have contributed to the decline in coronary heart disease including revised coding of death certificates, behavioral modification of risk factors, improvements in emergency and other hospital care, and use of new medications and surgery. The Rochester Heart Study contributed trend data covering the time before and during the decline in coronary heart disease mortality. At that time, medical care for residents of Rochester and Olmsted County was almost exclusively provided by the Mayo Clinic and Olmsted Medical Group. Under the Rochester Epidemiology Program Project supported by program project grants from the National Institute of General Medical Sciences and the National Institute of Arthritis and Musculoskeletal and Skin Diseases, the medical records of the Olmsted Medical Group and Community Hospital were indexed for retrieval by the same system as the Mayo Clinic. Those of other outside sources such as the University of Minnesota Hospital, Rochester State Hospital, and Veterans Hospitals which pertained to the occasional admission of Olmsted County residents were also added to the Mayo Clinic maintained diagnostic index. The resulting central diagnostic file gave virtually complete case assignments for diagnosed cases of myocardial infarction, sudden unexpected death, angina pectoris, and noncoronary heart disease.