Four computer discs containing the details of 17,990 current and former NHS staff in London have been lost in the post.
They were being sent between Whittington Hospital NHS Trust and McKesson, a firm providing payroll IT services.
The discs contained the names, dates of birth, national insurance numbers, start dates and pay details - but not bank account details.
Those affected are current and former staff at Whittington, Islington Primary Care Trust, Camden Primary Care Trust and Camden and Islington NHS Foundation Trust.
David Sloman, Whittington chief executive, said it was the first time information had been sent through the post. An investigation is being carried out and a member of staff has been suspended.
He said: "It is trust policy to send any such information by courier. To our knowledge, this is the one and only time that such information was directed through the post."
This latest loss of data follows the theft of six laptops from St George's Hospital, in Tooting, south London. They contained information about 20,000 patients, including 3,000 children.
The Unite union claimed in December last year that the personal details of 1,800 staff were accidentally sent to four companies by their health authorities.
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Whittington Hospital NHS Trust
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