WASHINGTON -- Companies' seniority policies almost always trump the demands of disabled employees, the Supreme Court ruled Monday in a 5-4 decision that continued the justices' trend of limiting the reach of the landmark Americans With Disabilities Act.

When choice jobs go to those with the most seniority, employers do not have to upend that system to accommodate a disabled worker such as former US Airways baggage handler Robert Barnett, the divided court ruled.

Barnett wanted a mailroom job that did not aggravate his back injury, but employees with more years on the job had dibs on the position. He sued under the ADA, the 1990 law guaranteeing fair treatment for the disabled on the job and elsewhere.

"The statute does not require proof on a case-by-case basis that a seniority system should prevail," Justice Stephen Breyer wrote for the majority. Rather, the assumption is that seniority systems take precedence, the court said.

Anything else could compromise the expectations or job security of all employees covered by a seniority system, Breyer wrote.

"We can find nothing in the statute that suggests Congress intended to undermine seniority systems in this way," the court said.

The court has ruled for employers and against a disabled employee each time it has examined the scope of the ADA in the workplace. Monday's decision is no exception, although the court did not give employers everything they wanted.

The court left a little wiggle room, ruling that employees covered by seniority systems remain free to try to show that their case is special. The court said it is entirely up to the employee to prove they deserve a break -- the employer gets the edge.

'Not a slam dunk'

The court gave examples of situations where a disabled employee might have a shot at bucking the seniority system, such as an employer's previous history of bending the policy.

"It's not a slam dunk, since they said employees could show there's exceptions to it," said Stephen Bokat, senior vice president and general counsel of the U.S. Chamber of Commerce.

Still, Bokat and lawyers who represent businesses said the ruling helps employers avoid situations where they might be sued by one employee for violating the ADA, and by other employees for violating seniority rules on behalf of the disabled worker.