Athletics chiefs have revealed drugs testing at the World Championships has turned up one "suspicious" result.
The IAAF has run its largest ever anti-doping programme in Japan
IAAF president Lamine Diack confirmed in Osaka on Sunday: "There's one case we have doubts about."
Spokesman Nick Davies would not say whether the test came from a medallist or if the athlete was male or female.
"It might not end up being positive," he said. "It means something that has to be checked into - something suspicious, doubtful."
It would be the first doping violation at the 2007 World Championships.
Two athletes had their performances annulled at the 2005 championships in Helsinki - hammer thrower Vladislav Piskunov of Ukraine and Indian discus thrower Neelam Jaswant Singh.
A total of 1,060 doping tests, including 510 blood tests and 550 urine samples, have been conducted on 926 athletes in Japan.