WASHINGTON (AP) _ Like a nightmare that returns years later, ''post-polio syndrome'' is forcing many former polio patients to return to the braces, wheelchairs and breathing aids that they stopped using decades ago.

The cause of the syndrome is not clear, but it seems to strike those who worked the hardest to overcome the crippling effects of their childhood nemesis.

Wendy Wilmer doesn't even remember having polio. She was only eight months old when it struck. But she spent her childhood overcoming its effects, enduring grueling hours of therapy, wearing braces and special shoes and restoring a crippled left leg through exercise.

''The idea was that if you gained a lost movement you would have it forever,'' she recalls. ''You would be fully recovered.

By the time she was 20, Mrs. Wilmer said she could run and dance and swim. She became an actress and performed regularly at community theaters. She also married and had children.

But just before she turned 30, recalls Mrs. Wilmer, ''I started falling down a lot.''

Within two years, she was unable to lift her left leg. And in the late 1980s, she was again wearing a brace and occasionally using a wheelchair.

Miriam Rappoport was only lightly touched with polio at age 4 and was free of her brace by the time she started school. She assumed, as did the experts at the time, that she was fully recovered.

Suddenly, when she was in her 30s, her legs began to bother her. They stiffened up and became progressively weaker. And there was the exhaustion.

''I was very tired all of the time,'' remembers Mrs. Rappoport. ''And it got worse and worse.''

For three decades, she went to a series of doctors without finding one who could diagnose the problem.

That changed when a niece suggested she go see Dr. Lauro Halstead at the National Rehabilitation Hospital. Mrs. Rappoport's condition was identified as post-polio syndrome and just putting a name on it was an emotional relief.

''At least I knew it was not in my mind,'' said Mrs. Rappoport. ''My regular doctor was very pleased to finally find out what the problem was. Most doctors don't know anything about it.''