A study at 10 Los Angeles high schools links e-cigarettes with later tobacco use.
University of Southern California researchers found that 14-year-olds who'd ever tried e-cigarettes were more likely than others to later try other tobacco products.
The study doesn't prove that electronic cigarettes are a "gateway drug" but some doctors say it bolsters arguments that the devices need to be strictly regulated.
The government-funded study was published in Tuesday's Journal of the American Medical Association.
National data show e-cigarettes have become more popular among teens than regular cigarettes.
The Food and Drug Administration has proposed rules that would ban the sale of electronic cigarettes to minors and would add the devices to the list of tobacco products it regulates.