Four years ago, San Francisco launched a grand experiment, becoming the first city in the nation to offer comprehensive health care to its growing ranks of uninsured. Stitching together two-dozen neighborhood health clinics and an array of hospitals, the city bet that two reforms ? emphasis on primary care and a common electronic enrollment system ? could improve outcomes and buffer the city against soaring health care costs. By many measures, San Francisco's effort to provide universal health care has been a huge success and has won national accolades. The initiative, Healthy San Francisco, has over time treated more than 100,000 city residents. But the city's grand plan has not solved the central problem dogging health care across the country: figuring out who pays for it.