The intrusion was detected in May 2010, The Wall Street Journal, which broke the story, reported. The FBI, which refused to comment, told the Chamber stolen information had been found on Chinese servers.
The hackers apparently targeted four Chamber employees involved in China and stole six weeks worth of e-mails.
Chinese officials in Washington and Beijing said computer hacking is illegal under the country's law. Geng Shuang, a spokesman with the Chinese Embassy, said blaming China "lacks proof and evidence and is irresponsible."
"What was unusual about it was that this was clearly somebody very sophisticated, who knew exactly who we are and who targeted specific people and used sophisticated tools to try to gather intelligence," David Chavern, executive vice president and chief operating officer of the chamber, said.
Chamber officials said there has been no evidence of misuse of stolen data.