In its ruling, the U.S. Eighth Court of Appeals in St. Louis made an exception in the case of self-funded plans used by some of the state's large employers, the Arkansas News Bureau said.
The state law requires health maintenance organizations to include any doctor as an approved provider once the doctor accepts HMO's conditions, including payment for medical services.
Supporters of the law argued that it will allow patients to choose their own doctors. Opponents said it prevents HMOs from holding down costs by promising a high volume of patients to selected doctors.
"This is a great victory for the patients of Arkansas," said a spokesman for Arkansas Attorney General Mike Beebe. "This ends 10 years of effort to get this law implemented."