However, the court said, the employer must allow the employee to present evidence of "special circumstances" that would make an exception to a senior rule reasonable in the particular case.

At issue in the case?US Airways v. Barnett?was whether requiring a company to alter its seniority rules to accommodate a disabled employee would place an "undue hardship" on the operation of the business.

The case involved a US Airways cargo handler, Robert Barnett. After injuring his back, he invoked seniority rights and was transferred to a less demanding mailroom position.

However, two other employees who were senior to Mr. Barnett also sought the mailroom position. Citing the ADA, Mr. Barnett asked US Airways to make an exception to its seniority system and allow him to remain in the mailroom job.

The company declined and Mr. Barnett lost his job. He then filed suit against US Airways, charging the company unlawfully discriminated against him by refusing his request.

In a decision written by Justice Stephen Breyer, the majority opinion said that a seniority system provides important employee benefits by creating expectations of fair, uniform treatment. Nothing in the ADA, the majority said, suggests that Congress intended to undermine seniority systems.

Thus, the court said, it will generally be sufficient for an employer to show a violation of seniority rules in order to determine that a requested accommodation is not reasonable.

But the court said there may be special circumstances that might alter the expectations of fair, uniform treatment. For example, the employee may be able to show that the company retained the right to change the seniority system unilaterally and exercised that right frequently.

Under this or other special circumstances, the court said, the employee may be able to prevail. But the court said the burden is on the employee to make the showing of special circumstances.

In a dissent, Justice Antonin Scalia criticized the majority's opinion. He said the opinion leaves the question of whether a seniority system must be disregarded "in a state of uncertainty that can be resolved only by constant litigation."

But in a separate dissent, Justice David Souter attacked the majority's opinion from the other direction. He said that nothing in the ADA insulates seniority rules from the statute's reasonable accommodation requirement.