The queue for Boom Chicago stretches right across the courtyard, so I ask around to find out why this show has become so popular. "I heard about if from a friend," says the French girl next to me. A New Yorker has a similar explanation: "I met one of the guys."

Odd. But there again, word of mouth is so much a part of Boom Chicago's act that it is appropriate that it has also become their advertising. This is improvised comedy that begins when five people from the audience are questioned and filmed. Their "personalities" are then adopted by the cast. Big on comic interplay and rapier-like of wit, the five Americans take us through a series of adventures. Shouted suggestions are then woven into the storylines. However the appeal of Boom Chicago is Coming is very mainstream. Pick Ups and Hiccups, also at the Pleasance from the same company, is the one to queue to see.

Till Sunday. Box office: 0131-556 6550.