The Internal Revenue Service says 99,123 taxpayers didn't receive their checks because they had the wrong addresses on them. The returned checks average $1,547 apiece.
In Los Angeles County, $7 million in checks to more than 4,000 taxpayers went undelivered.
Meantime, in Riverside County, nearly 700 people have tax refunds waiting to be claimed.
Taxpayers hoping to claim their refund can click on the "Check on Your Refund" link at www.irs.gov, or call 1-800-829-1954.
The Associated Press contributed to this report.