In an annual financial report, the UAW said its membership fell to 431,037 last year, the Detroit Free Press reported Wednesday.
In the year, 33,873 members left the union, which peaked in membership at 1.5 million in the early 1970s.
UAW President Ron Gettelfinger has said the numbers are deceiving, claiming there are 513,000 active members and more than 575,000 retired members.
The report said the union spent $10.6 million on lobbying last year out of gross receipts of $315.7 million.
Putin thinks Obama would save him if he were drowning
Rosie O'Donnell unveils nearly 50-pound weight loss