Thieves targeted over 50 work vans in Cardiff and the Vale last month, according to South Wales Police.
Barry was the worst affected area in the region, followed by the Fairwater area of Cardiff.
It's believed the suspects have been mostly targeting the vehicles for power tools.
Detectives say they're investigating the recent spate of thefts and are reminding van owners to be on their guard.
A spokesperson for South Wales Police said: "We understand that having your van or work vehicle stolen, broken into and contents stolen can be very distressing. These recent thefts are being investigated."
"When you’re parking your van, even for short periods, please think carefully about where you are stopping. Parking with the rear or side doors against a wall or very sturdy railings will help to ensure that they can’t be prised open."
"Please consider removing tools and other items from vehicles overnight."
More security advice can be found on the Secured by Advice website.