A California man is suing the infidelity website Ashley Madison and its parent company for negligence, saying he has suffered emotional distress.
The lawsuit, filed in US District Court in Los Angeles against Ashley Madison and Avid Life Media, seeks class-action status.
It accuses the companies of failing to adequately protect clients' personal and financial information, and seeks unspecified damages.
Data from about 37 million people using the Ashley Madison website was released online in August by a group calling itself The Impact Team.
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The lawsuit claims this could have been prevented if the company had taken "necessary and reasonable precautions to protect its users' information, by, for example, encrypting the data".
The group had threatened to release the data if the website was not closed down.
The information dumped online includes the email addresses of government officials and senior executives who were apparently using the website.
The US lawsuit comes just days after police in Canada said two suicides in Toronto may have been linked to the Ashley Madison hack.
Bryce Evans, staff superintendent of the Toronto police, told reporters on Monday: "As of this morning, we have two unconfirmed reports of suicides that are associated because of the leak of Ashley Madison customers' profiles."
He said that other users had become the victims of blackmail and other "spin off crimes" from the hack.
