High-value cars and work vans are being targeted by gangs who break into homes to snatch keys in Berkhamsted and Tring, police said.
Five vehicles worth up to £15,000 including a BMW and an Audi have been stolen since the start of August.
Mike Pryce, new Herts Police Chief Inspector for Dacorum, said: "These gangs will target people based on the value of their car.
"They c
ould also fill your car up with your possessions."
According to police records, the spate of car thefts started when a blue Volkswagon Beetle was taken by thieves who stole keys from a home in Castle Hill Avenue, Berkhamsted between 9pm on July 31 and 9.30am on August 1.
The crooks also took front and back door keys and two laptop computers.
Then between 9am on August 10 and 2pm on August 11, burglars broke into a home in Park View Road, Berkhamsted and from a cupboard took keys to a silver BMW which was stolen.
A CD player, wrist watch and silver cuttlery were also taken.
At about 3.30pm on August 12, thieves entered an unlocked door at a home in Billet Lane, Berkhamsted and stole keys from a table to a Ford Transit van which was taken - the vehicle had 'flat-pack' packages including furniture and rabbit hutches inside.
Then in Tring, between 7.30pm on August 11 and 7.30pm on August 12, burglars entered a home in Beaconsfield Road through an unlocked door and stole keys to a blue Audi estate which was taken.
Finally, at about 3.10pm on August 5, thieves reached into the porch of a home in Bulbourne Road, Tring and took the keys to a blue Ford Transit van which was stolen.
CI Pryce said: "The message is don't leave your keys in the hallway because someone will break in, take your keys and take your car.
"Leave your car in a garage if you can, and obviously if you see someone acting suspiciously, call the police."
The latest police figures show theft of cars is down by almost 60 per cent in Berkhamsted since last year.
The statistics show there were 14 cars stolen from April to July 2007, compared with just six in the same period this year.
In Tring, the figures show car thefts are up by almost 13 per cent, with 125 vehicles stolen in 2008 compared to 111 in 2007.
Anyone with information about car crime in Dacorum is asked to call Herts Police on 0845 3300222.
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