The teen jumped onto the trunk of one of the cars, which was moving and then subsequently hit the second car, Pound Ridge Police Chief David Ryan said.
"The 17-year-old was thrown from the vehicle and sustained serious injuries," Ryan said.
Ryan said that the teenager was taken to Westchester Medical Center in Valhalla and was "in serious but non-life threatening condition with a serious head injury."
A 17-year-old Mount Kisco resident who was driving one of the vehicles was issued a traffic summons, Ryan added.
Pound Ridge police are asking members of the public with information about the incident to call the local department at 914-764-4206.
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