Loudly. And, as it turns out, successfully.

The leader was Marcy Pedas Sigler, who had lived on Fifth Avenue since 1973 — until she opened her mail last week and discovered her once-tony address had been changed to a pedestrian " 1080 Museum Mile."

"The last cabdriver I had couldn't find Lincoln Center," Ms. Sigler complained. "Can you imagine trying to find 1080 Museum Mile?"

Well, no. But the folks at the post office obviously could.

"Dear Postal Customer," began the address-switching card sent to people living from 82nd Street to 104th Street, "To ensure accurate and timely delivery, it is very important to make sure all the mail sent to you displays the EXACT address as it appears here."

The city started using the name Museum Mile for the 22-block stretch in 1981. There are 10 museums on Museum Mile " and all use a Fifth Avenue address.

While the city frequently provides street aliases "Fashion Avenue for Seventh Avenue, Avenue of the Americas for Sixth Avenue" people move to Fifth Avenue for a reason.

It's known internationally, easily identifiable to folks from Tennessee to Tokyo. And the location - opposite Central Park, amid the museums — makes it one of the city's most desirable addresses.

Ms. Sigler, the head of her building co-op board, received nine calls from angry tenants. Doctors, businesses and residents began computing the cost of new business cards and stationery.

When they finished the math, it added up to some angry people. The post office said Wednesday that it had moved to defuse the controversy by allowing residents to use either address, Museum Mile or Fifth Avenue.

"If there was any confusion caused by our card, we apologize," said postal spokeswoman Pat McGovern. "There is another letter going out to clarify things."

The outcry from la créme de la upper East Side came last week after they received address cards from the Postal Service advising them that Fifth between 79th and 106th Streets now would bear the street name Museum Mile.

The notices, incidentally, arrived U.S. Post-haste — 14 years after the city dubbed the 27 - block stretch Museum Mile.

The name change was inspired by the presence of 10 museums on the stretch — all of which use a Fifth Avenue, address.

"Fifth Avnue is Fifth Avenue " said Harold Holzer, spokesman for the Metropolitan Museum of Art. "We will remain 1000 Fifth Avenue "

Marcy Pedas Sigler, who has lived in a Fifth Avenue co-op since 1973, said she received nine calls from angry residents in her building.

"The last cab driver I had couldn't find Lincoln Center," Sigler said yesterday. "Can you imagine trying to find 1080 Museum Mile?"

Pat McGovern, spokeswoman for the Postal Service, said the cards were meant to clarify that both Museum Mile and Fifth Avenue were acceptable addresses.

"There was a misunderstanding. Both addresses are valid" she said, explaining that Museum Mile for Fifth could be used the way Fashion Avenue is used for Seventh. After all, Fifth Avenue by any other name might as well be the Bowery.