unpaid by poor patients — that is the equivalent of $30,000 in routine emergency-visit charges that would otherwise be written off as charity.

La Clínica charges $85.50 per consultation; low-income patients are charged less.

“If we didn’t have La Clínica, we’d be in much worse shape,” says Angie Hammons, Sutter’s emergency-room manager.

One of the clinic’s new patients is Evelia Lopez, 51, who had been visiting the emergency room to treat chronic back pain after a slip at work. About two-fifths of the clinic’s patients are Hispanic, while about a quarter of the patients are African-American; one-fifth are white.

Along with their medical history, new patients are asked their income to det...

unpaid by poor patients — that is the equivalent of $30,000 in routine emergency-visit charges that would otherwise be written off as charity.

La Clínica charges $85.50 per consultation; low-income patients are charged less.

“If we didn’t have La Clínica, we’d be in much worse shape,” says Angie Hammons, Sutter’s emergency-room manager.

One of the clinic’s new patients is Evelia Lopez, 51, who had been visiting the emergency room to treat chronic back pain after a slip at work. About two-fifths of the clinic’s patients are Hispanic, while about a quarter of the patients are African-American; one-fifth are white.

Along with their medical history, new patients are asked their income to determine what pay on a sliding fee scale.

As in emergency rooms, patients aren’t asked about their immigration status.

Costs at such primary-care centers are probably 10% to 15% the cost of treatment in a hospital emergency room, says Paul Mango, head of the health-care practice at McKinsey & Co.

Residents have since complained to a 19-member county-appointed watchdog group about taxpayer money La Clínica going to health care for people living in the U.S. illegally. Neither the clinic nor the Sutter emergency room ask people their immigration status.

“All we can ask them is their name, date of birth and chief complaint,” says Ms. Hammons, the Sutter emergency-department manager. “Heavens, we don’t deny anybody treatment. You are required to see anyone who shows up at the emergency department.”

Mike Reagan, a Solano County supervisor who originally voted for the clinic’s funding, now says the facility should erect a “firewall” to prevent taxpayer money from going to illegal immigrants. “I’m not in favor of rewarding illegal behavior in any form,” he says.

The report from the watchdog, released three weeks ago, recommends that Solano County require that public contributions to the clinic “be limited to serving only Solano County resident...

“All we can ask them is their name, date of birth and chief complaint,” says Ms. Hammons, the Sutter emergency-department manager. “Heavens, we don’t deny anybody treatment. You are required to see anyone who shows up at the emergency department.”

Mike Reagan, a Solano County supervisor who originally voted for the clinic’s funding, now says the facility should erect a “firewall” to prevent taxpayer money from going to illegal immigrants. “I’m not in favor of rewarding illegal behavior in any form,” he says.

The report from the watchdog, released three weeks ago, recommends that Solano County require that public contributions to the clinic “be limited to serving only Solano County residents who have proof of citizenship or legal residency.”

The county’s board of supervisors and health director have 90 days to respond. County health director Patrick Duterte says he is bent on keeping the clinic open.

“My position is that to have a healthy community we can’t have a subset of people who don’t have access to health care,” says Mr. Duterte. “It’s bad public-health policy.” Health experts say that giving undocumented immigrants medical care can prevent the spread of illnesses.

Meantime, the clinic has extended its hours to keep pace with swelling demand. “We’re swamped,” says Monique Sims, the clinic’s manager.