Honda is warning 2.2 million consumers that their email addresses and vehicle identification numbers have fallen into the hands of hackers.
A further 2.7 million Acura owners are being told their email addresses were stolen.
No financial information was included in the hacked customer lists.
In its warning, released from America Honda Company, Honda stresses that the danger to customers is spoof emails or spam that could try to trick them into providing further information such as credit card numbers.
"Be cautious of unsolicited emails requesting personal information. Often, these communications can look official," the automaker points out on a special web page for affected owners.
"If ever asked for this information, you can be confident it is not from us," the warning states.
The security breach came from a third-party company that was handling data for the automaker.
Customers whose information was hacked have already been contacted by Honda.