The artists formerly known as ballet dancers

"What means this word Billboards?" queried my French guest at the Festival Hall, 20 minutes into the Joffrey Ballet of Chicago's rock ballet, Billboards. Um, how to explain. It's American for those huge adverts they have by the side of the road. "Ah voila," she came back. "That fits. It is saying two dimensions. It is saying popular. And it's saying it wants very much to sell something." Got it in three, Michele.

The story of Joffrey's venture into global pop culture could have been one of those rags-to-riches heart-warmers. Revered 40-year-old classical ballet company risks going to the wall for lack of interest in Aurore, Giselle & Co. Along comes The Artist Formerly Known as Prince to his first Joffrey gala, loves the show, rushes off home and re-mixes his greatest hits as a thank you. The Joffrey builds a ballet on them. Its fortunes revive.

So Aurore and Giselle return to the boards triumphant? Not in this story. Terrified of killing the golden goose, the company is now condemned to dance this trash till it drops. For trash it is from start to finish - of the emptiest, tinniest, most readily discardable kind.

As Prince's "Sometimes it Snows in April" sets up its virile disco thump, rhinestone-twinkly girls and boys get up on their toes and archly parade their wares. You thought ballet dancers were sissy? bellows Laura Dean's choreography - then get this. Pairs of girls pirouette prettily on pointe to meet each other then erupt in a raucous bump'n'grind routine. The boys launch an offensive of extreme balletic leaps and mid-air splits. Who can jump the highest, the furthest, the fastest? But if this is the ballet Olympics, no one deserves a medal. I've seen better.

As Prince (on tape) reaches his umpteenth climax, the dancers join hands to whoop their way through an Okey Cokey of high-school aerobics - Pan's People meet the Kids from Fame. The piece groans under a frenzy of false ecstasy, like a teenage party that's been allowed to go on too long. What it's trying to prove of course is that ballet can be fun, sassy and sexy. The tragedy is that American audiences apparently need this underlined in red. Dance-followers in Britain will know that there was more sex in Kenneth MacMillan's big toenail than there is in the whole of Las Vegas. Or, if we're talking rock-cred, then in Ashley Page's bit of rough in Fearful Symmetries set to John Adams ... or Christopher Bruce's Rooster that makes hay with the Rolling Stones. Billboards protests too much.

With appalling aptness the Laura Dean piece ends with the company falling flat on their faces. As indeed do the three other choreographers who contribute to this unholy hybrid - Charles Moulton, Margo Sappington and Peter Pucci. I hope we were meant to snigger at "I Want to Melt with U" - an erotic underworld in which a girl in a gold bra tickles a branding iron over the sensitive parts of various blokes in orange wigs while the chorus simulates urgent copulation.

But we were certainly meant to keep a straight face during "Purple Rain", a melodramatic solo in the mould of Pierrot Lunaire, even when the desperate clownette stepped out of her costume and got hoisted like a dead Christ onto the shoulders of a pack of hunky dudes. Dramatically slack, the piece was as empty as the dancer's discarded clothes and, frankly, I was embarrassed. If it's ballet you fancy, hie thee to a ballet. If it's an appropriate response to Prince's juicy lyrics, stick on a video of His Purple Highness himself.

Royal Festival Hall, SE1 (0171 960 4242), to Sun 8 Sept.