VDR, the German travel managers' association, has demanded a clarification from Lufthansa over how it uses passenger travel records.
The association has asked the airline whether it regards such information as confidential and how many thousands of its employees have access to it.
The letter, written by VDR president Michael Kirnberger and his vp Ralph Rettig, was sent this week to Lufthansa's president and ceo Wolfgang Mayrhuber.
The concern of VDR members rose when Lufthansa admitted it had searched through passenger records to find out which of its supervisory board members was leaking stories to a journalist.
Lufthansa said examination of flight records was legal and as such leaks were against the German Stock Companies Act, it had to find out who was responsible.
The airline denied it had spied on or investigated the journalist.
The searches took place in 1999-2000 and the supervisory board member has since left the airline.
In the letter to Mr Mayrhuber, the VDR it was deeply concerned that passenger data was being used for purposes for which it was not intended.
It such the confidentiality of such data was protected both by German regulations and European data protection guidelines.
The VDR said that statements by Lufthansa spokespersons indicated that the data would be less well protected than that in telephone directories.
The letter called on Mr Mayrhuber to say which data protection regulations Lufthansa followed, how well protected the data was and how VDR members could regain confidence in Lufthansa's handling of data after its recent "carefree" approach.