Attendees lining up outside Desigual in Lyon for the Undie Party // Desigual

Barcelona-based apparel brand Desigual is hosting the San Francisco edition of its infamous Undie Party Friday, March 2.

The first 100 people to enter Desigual’s Union Square location on March 2 clad only in their underwear get to leave with a Desigual outfit, any one top and one bottom, without paying a penny. There are no price caps beyond one top and one bottom, though the new Spring Summer 2012 is excluded.

These 100 underwear-clad shoppers will have the store to themselves for an hour and the only entry rule is that Undie Party-goers must be 18 or older with photo ID. Those in line after the first 100 (in their underwear) will receive a 50% discount on same-day purchases.

Rain or shine, the line for Desigual’s Undie Party starts at 7am and doors open at 9. Of course, smart San Franciscans may want to wear warm clothing in the nippy morning and strip down once qualifications start (8am).

An Undie Party attendee shopping for clothes at Desigual

The Powell Street Desigual opened roughly a year ago and is the Bay Area’s only location. To date, Desigual has never had more than one Undie Party per store, so this could be San Francisco’s first and only Undie Party.

Desigual’s Undie Party (otherwise known as “Come half naked, leave fully dressed) tour kicked off in Madrid, Spain in January, followed by Seville, then Lyon, France. San Francisco will host the fourth and only U.S. Undie Party scheduled so far. San Francisco’s also appears to be the last party scheduled for the year.

Since the first U.S. store opened in New York in 2009, the Spanish company that sells men’s, women’s and children’s clothing and accessories has been expanding rapidly stateside, adding locations in LA, Las Vegas, Miami and two more in Manhattan.