America's Best Hospitals, an annual ranking of the country's elite medical centers, is a tool for patients who need medical sophistication most facilities cannot offer. Unlike other rankings and ratings that grade hospitals on how well they execute routine procedures like outpatient hernia repair or manage common conditions like low-grade heart failure, the U.S. News approach looks at how well a hospital handles complex and demanding situations—replacing an 85-year-old man's heart valve, diagnosing and treating a spinal tumor, and dealing with inflammatory bowel disease, to name three examples. High-stakes medicine.

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This year, the 20th for Best Hospitals, institutions are ranked in 16 specialties, from cancer and heart disease to respiratory disorders and urology. A total of 4,861 hospitals were considered; 174, or less than 0.4 percent of the total, were ranked in even one of the 16 specialties.

In 12 of the 16 specialties, those in which quality of care can spell life or death, hospitals were scored on reputation, death rate, patient safety, and care-related factors such as nursing and patient services; the 50 highest scorers were ranked. Scores and complete data for unranked hospitals are available as well. In the other four specialties—ophthalmology, psychiatry, rehabilitation, and rheumatology—hospitals were ranked on reputation alone, because so few patients die that mortality data don't mean much.

Here are a few of the details: Reputation, which counted as 32.5 percent of the score, was based on three years of specialist surveys—a total of almost 10,000 physicians were asked to name five hospitals they consider among the best in their specialty for difficult cases, without taking into account cost or location. A mortality index, also 32.5 percent of the score, indicates a hospital's ability to keep patients with serious problems alive. Patient safety, new this year, made up 5 percent of the score; it indicates how well a hospital minimizes harm to patients. And a group of other care-related factors, such as nurse staffing and available technology, accounted for the remaining 30 percent.

Of the 174 hospitals that are ranked in one or more specialties, 21 qualified for the Honor Roll by earning high scores in at least six specialties. This demonstrates unusual breadth of excellence. Johns Hopkins Hospital tops the list, as it has every year from 1991 on. (The Mayo Clinic was No. 1 in 1990, Best Hospitals' first year.)

Hospitals are listed by total points. A hospital got 2 points if ranked at or close to the top in a specialty and 1 point if ranked slightly lower.