The Deadheads dance.

They dance before the Grateful Dead show. They dance afterward, partying around the cars and buses they have driven from places spread across America. At the Merriweather Post Pavilion last night, with a breeze blowing through the wooden pavilion, they were a kaleidoscope, loose-limbed and tie-dyed, writhing on the lawn and between rows of seats. They moved to personal rhythms throughout the outdoor theater.

They dance, they party, they wait -- ever vigilant -- watching to see what their band will do.

"On any night, anything can happen," said a Deadhead, a young accountant from Alexandria.

Every show is different, it is said, like snowflakes. Perhaps that's why even in their 20th year the Grateful Dead continue to draw legions of followers from everywhere to every show. A quick glance at the cars and trucks mired in the traffic jam on U.S. 29 outside Columbia, Md., reveals the distances people travel. A Blazer from Arizona, a Volvo from Oregon, a slew of cars from nearby states like Connecticut, Pennsylvania and New York.

Lawrence Feinberg came from California. Why all the way from California? "There is nothing I'd rather do than share a Grateful Dead show with good friends," he said as he stood on the side of the road with dozens of others, looking for tickets among the slowly passing cars.

Almost since the beginning in 1965 and 1966, the Grateful Dead have been on the road, first gaining notoriety by giving free concerts -- dubbed the Acid Test Rock Dances -- in San Francisco's Golden Gate Park. Even before the 1967 release of their first album the band had a cult of followers enamored with their music, the life style in their Haight-Ashbury home and LSD.

In 1974 the Grateful Dead formed their own record company, one of the first major rock bands to do so. In 1984 they did almost 60 shows throughout the United States. Although their music will be heard this fall on the 1980s TV version of "The Twilight Zone," they haven't released an album in about five years. Nor have they made any videos, but the Deadheads don't mind; hundreds of bootleg tapes, documenting every show, are available.

The extraordinary success of this band, described as "all rock 'n' roll, but looking to the roots," is as difficult to explain as why their "space jams" -- those rambling instrumentals -- are so appealing. A Deadhead from New York City suggests that people are drawn to the concerts, some to every show on a tour, because "every show is different but everybody comes back for the same thing. I can't explain it," explains a New York City man with a wispy beard and kinky blond hair. "Just listen and look."

The same people make up the band now as in the days of the Acid Tests. Almost. In 1973, after years of playing keyboard and of heavy drinking, Ron (Pig Pen) McKernan died. Pig Pen was replaced by Keith Godchaux, who died in a car accident and was succeeded by Brent Midlin.

As "Fat Man" Jerry Garcia's guitar notes floated on the warm damp air, the carnival raged. Bob Weir's vocals led the rest of the band -- Phil Lesh, Bill Kreutzman and Mickey Hart -- through a set of sometimes blues, sometimes country, mostly rock 'n' roll. The Deadheads celebrated the music and the scene like so many loonies in a bin: dancing with wired eyes and wide grins. It is clear that they love their band. Just as clear is that LSD and marijuana, so much a part of the shows, are not out of vogue. "If you're doing a dose (of LSD), we're talking serious energy," said a young man with an impish smile. He has traveled from Ipswich, Mass. and is wearing a large blue floppy hat that doesn't quite cover his long, tight braids.

One Deadhead at the Philadelphia shows who, along with 23 or so Californians had followed the band in an old bus (green and brown with "Nairobi to Mombasa" painted on the side), said that too much is made about drug use at the shows. "Usually it's the tourists who come to the shows and do doses [of LSD]," said the 19-year-old in braids with shiny braces on her teeth. "The people who tour don't trip all the time -- it gets in the way."

"When you're on tour," chimed in her fellow traveler, "there's too much to be done, with the bus and all."

"The people make the band," said Andy Blum of Fort Lee, N.J., a rider on another old bus. "The band doesn't make the people."

Inside, a man with a long beard, standing in the mass of people on the floor near the sound board, said, "They have continued to be, for the 18 years I have seen them, one of the most experimental and professional bands around."

Every concert is taped by dozens of fans, sometimes by more than 100, intent on getting the best recording possible to trade for tapes of shows they miss. Some tapers spend as much as $2,000 on their equipment, boasting of the hundreds of live tapes they own and loan. "We don't actually trade tapes," said one taper, "we loan them for a long time."

Eileen Law, the head Deadhead who has been with the band since 1972 and now works in their San Francisco office, said the tapers are a different breed. "I love collecting tapes, but I can't keep up. I just got tapes from the New Year's shows-I can't believe the energy of people on the East Coast. The clapping, the singing along.

"And as far as the appeal of the band, it's the music, which varies each night. People don't want to miss a song they haven't heard before, or one done really well. They go because they feel they might miss something. Even lawyers schedule lawsuits around the Dead tours."

The two Merriweather Post shows were sold out, like most Grateful Dead shows. That's 26,000 tickets. And the shows in June at the Greek Theater in Berkeley, Calif., beginning the band's 20th-year celebration tour, were sold out months in advance. One couple even flew from West Germany to be at the shows in Berkeley. Like many Deadheads they will be following the band for the entire summer tour. By Law's reckoning there are more than 100,000 registered Deadheads who receive newsletters and promotional information. But she admitted the count is conservative. She's months behind in the mail.

The Deadhead is by all appearances an anachronism. A hippie, in the '60s, the '70s, and now in the '80s, hanging on to a go-with-the-flow attitude. Sharing and kindness are important, a matter of pride. And there is much to share. Deadheads in general are not poor. Many people at the shows have followed the band for more than a decade, their hair showing signs of gray, their faces more responsibility. But a large portion of the Deadheads are in school, or just out of school, carrying on a tradition that doesn't seem to be fading away.

The shows can be good or bad. But something ties one show to the next, meshes the years and the geography together. Search in the parking lot outside a Dead show. If the concert hall is a carnival, the parking lot is a picnic where people come early and stay late, partying in the great American tradition. Different concert tapes play every 10 feet, hawkers sell tie-dyed clothes and, with an open ear, you may hear the acid dealers cruising through announcing their wares.

For the hungry there's food to be found. There's never a shortage of beer. Should you be interested and lucky, you might find someone throwing a frisbee.