Urgent-care centers have been gaining ground across the country recently as an attractive medical option for time-pressed families trying to avoid spending hours in a hospital ER or days waiting for a doctor's appointment. The centers offer walk-in medical services and extended hours to customers with non-life-threatening medical problems. Doctors provide the care, assisted by nurses, and generally X-ray and laboratory services are available. Most centers are open 365 days a year, and insurance policies cover most services. This convenience-oriented format is getting a boost as hospitals and private firms build new centers, responding in part to new competition from retail clinics.