TULSA, Okla. — When Lou Ellen Horwitz first learned that a gas station company was going to open a chain of urgent care clinics, she was skeptical.

As CEO of the Urgent Care Association, Horwitz knows the industry is booming. Its market size has doubled in 10 years, as patients, particularly younger ones, are drawn to the convenience of the same-day appointments and extended hours offered by the walk-in clinics.