WASHINGTON — In-store medical clinics like those at Walmart, having established a beachhead with relatively healthy patients looking for convenient, low-cost care for simple problems, are going after a bigger prize: the millions of Americans with costly illnesses such as diabetes and heart disease.

Just as Wal-Mart Stores and other retailers shook up the prescription drug business by offering $4 generic drugs, the industry now aims to apply its negotiating and marketing clout to problems that vex consumers and the health sector: unpredictable costs, a lack of primary-care doctors and inefficient management of chronic illnesses, whose costs drive the majority of health-care spending.

“It’s sad that the existing health-care establishment has not figured out a way to make primary care affordable and accessible,” said Jerry Avorn, a professor of medicine at Harvard. “We should not be surprised if someone outside of our world comes in and does it for us.”

Last week, Wal-Mart’s ambition to become the nation’s largest provider of primary health-care services became known when a confidential document the retailer sent to some of its strategic partners leaked out. The request for information sought partners who could help Wal-Mart in a variety of areas, including monitoring patients with diabetes, asthma, high blood pressure, heart disease and obesity.

Wal-Mart backed away from parts of its own document, saying it did not intend to build a “nationally integrated, low-cost primary care platform.”

But clearly, Wal-Mart and other retailers are looking for ways to expand services at their in-store clinics.

CVS Caremark, the largest operator of in-store clinics with nearly 550, and Walgreen, have set up programs to help diabetics monitor and control their condition. Weight-loss programs and counseling are available at some retail clinics. Truck drivers can get their mandatory federal health exams at more than 600 such in-store centers.

“There are real savings to containing the cost of the chronically ill in this country,” said Helena Foulkes of CVS Caremark, which offers in-person and telephone conversations with pharmacists to diabetic patients enrolled in its “Pharmacy Advisor” program. Some employers also send workers to the firm’s MinuteClinics for blood tests and other health screenings. “More and more of clients are actively looking for wellness programs and they see retail clinics as one element.”

In part, the clinics see a pure business opportunity based on consumer convenience and cost savings, which they can market to the public, employers, insurers and hospitals. Costs are 30 percent to 40 percent less than similar care at a doctor’s office, and 80 percent cheaper than at an emergency room, according to a study in published this year in the American Journal of Managed Care.