Triathlon is one of the most difficult to predict of all Olympic sports and Sydney 2000, when it made its Games debut, perfectly illustrated the point.

Australia were expected to dominate the women's race, having provided the first five home in the previous year's world championships, yet they managed only a silver as Swiss duo Brigitte McMahon and Magali Messmer took gold and bronze respectively.

In the men's event, Britain's four-times world champion Simon Lessing was the pre-race favourite but failed to mount a serious challenge.

Instead, Canadian Simon Whitfield swept to victory, ahead of surprise silver medallist Stephan Vuckovic of Germany and Czech Jan Rehula.

The usual sporting vagaries of form, fitness, health and luck are multiplied by three as each of the disciplines -- swimming, cycling and running -- present their own challenges.

The two transitions, when athletes change kit at maximum speed, add to the mix -- as Briton Stephanie Forrester found in Sydney when she got stuck in her wetsuit and lost so much ground on the leaders that her race was effectively over.

With the bike leg also including the risk of mechanical failure, puncture or crash, it is easy to see why the bookmakers are reluctant to stick their necks out on any of the big names.

Consequently, when the women line up to race on August 25 and the men the next day, virtually everyone in the field will be a potential medallist.

McMahon, back at the top after having a third baby, will be back to defend her title but Sydney silver medallist Michellie Jones will not, after controversially being overlooked by the Australian selectors.

Her compatriot Emma Snowsill, the 2003 world champion, is another notable absentee from the women's race, piling the pressure on the Australian trio of Rina Hill, Loretta Harrop and Maxine Seear to get among the medals.

The United States should also mount a formidable challenge as Barbara Lindquist, Sheila Taormina, winner of this year's world championships and a former Olympic gold medallist in the pool, and Laura Reback currently fill the top three places in the world rankings.

Canada's Carol Montgomery, the 2002 Commonwealth Games gold medallist whose broken wrist in a crash in the Sydney triathlon ruled her out of a planned track assault on the 10,000 metres, will be strongly fancied if she can stay in touch on the bike leg.

Her compatriot, Whitfield, who also won the Commonwealth Games gold in 2002, will be there to defend the men's title.

Australia are again strong through the formidable Peter Roberson, Greg Bennett and Simon Thompson while New Zealand, led by world champion Bev Docherty, could also feature.

Dmitry Gaag of Kazakhstan, fourth in Sydney, is back in hot form while Spain's Ivan Rana, fifth in Sydney, world champion in 2002 and twice second since then, has a proven big-race pedigree.