Independent physicians who set up and own stand-alone clinics have historically driven growth in the urgent-care industry. But hospitals are increasingly turning to the model to help grow their footprint and boost referrals. According to data gathered by the Urgent Care Association of America, hospitals now own about a quarter of the country's 9,000 urgent-care clinics, up from a marginal presence just five years ago. Private physicians own slightly more than one third of clinics. Independent and corporate-owned clinics are behind a regional boom in urgent openings in recent years, though several hospitals have gained traction in the industry.