Ever since I was born on Halloween, I've enjoyed a good costume party. I usually start planning in August, lying on a hot sunny beach, what I'll wear that chilly night in late October. Today, though, I have a more urgent costume concern-what to wear for the 95th annual ING Bay to Breakers foot race, this Sunday, May 21.

As much a party as a competition, the 12K (7.46-mile) event draws roughly 70,000 runners and walkers who cross San Francisco, heading west in a steady stream of athletic determination mixed with kooky wigs, masks, Elvis outfits, high heels, flip flops, body paint, floats, and for a small (and presumably chilly) contingent, birthday suits. The official 12K National Championship event must be the zaniest race recognized by the USA Track & Field Association, and the only ambulatory carnival that can qualify American runners for the Olympic trials if their race times are fast enough.

Founded in 1912 as the Cross City Race, the original event drew a mere 200 participants. It peaked at 100,000 in 1986, earning it a Guinness Book world record. This Sunday's edition commemorates San Francisco's great earthquake of 1906, since the race was created in part to help the city recover from that disaster. The costume tradition emerged in 1940, when a participant dressed as "Captain Kidd" finished last, and the first-ever female runner, Bobbie Burke, disguised herself as a man in order to compete. The race was renamed Bay to Breakers in 1964, and added the ING prefix after picking up corporate sponsorship. The race welcomes everyone from casual walkers to serious athletes, including occasional runners like me. Slow pokes must stay in the back, along with the centipede teams (tandem runners joined by their outfits), floats such as the mobile Tiki Bar, baby joggers, and any other wheeled or over-sized objects or costumes. One well-loved team is the Spawning Salmon, who runs the race backwards wearing fish suits.

Official race rules state "No Alcohol or Nudity," but the latter prohibition so far hasn't dissuaded the race regulars known as Bare to Breakers^TM. This clothing-free delegation runs every year, and is raising money for the Leukemia & Lymphoma Society, as many teams do.

The race morning traditionally starts with a tortilla toss, in which thousands of tortillas are flung into the air. At 8 a.m., the first runners begin weaving their way from the city's edge at the Embarcadero, near the business and tourist district, through the newly trendy Hayes Valley neighborhood, then uphill towards Alamo Square and the famous Victorian row houses known as the Painted Ladies. The final stretch cuts through Golden Gate Park before stopping just shy of the Pacific Ocean.

Free talks at an accompanying expo on Friday and Saturday include "Why Go Vegan?" with natural vegan bodybuilder Kenneth Williams, two panel discussions on women runners, and other events about nutrition and exercise for kids. On race day, New Orleans pop group Better Than Ezra headline a free concert in Golden Gate Park dubbed "ING Footstock," after the official costume judging sponsored by Southwest Airlines (winners receive free trips to San Diego, Los Angeles, Phoenix, and Las Vegas).

You can register in person as late as this weekend ($40; $35 for children), or you can join the 100,000 spectators. If you go, be sure to wave at the guy who looks like he's wearing last year's Halloween costume.

Photo: San Francisco Chronicle