US government lawyers want to see records of Lance Armstrong's cancer treatment to find out if doctors knew he was doping in 1996.
Armstrong, 43, confessed to taking performance-enhancing drugs in 2013, having been stripped of his record seven Tour de France titles.
The US government is suing Armstrong for millions of dollars of sponsorship paid to his team by US Postal Service.
The request is an attempt to "harass" Armstrong, according to his lawyers.
Armstrong was diagnosed with testicular cancer in 1996, but returned to claim seven successive Tour titles from 1999.
The Texan has admitted to doping from before his diagnosis, but evidence that doctors knew about it could be used to support the theory that there was a wider conspiracy to hide his cheating from federal backers.
Armstrong is banned from professional cycling for life but took part in a charity ride on the route of this year's Tour de France, a day ahead of the main race.