As of July 1, 2008, California drivers must use hands-free technology when talking on a mobile phone, and drivers under age 18 may not use a mobile phone at all while driving. This study examines how fatalities changed in states after a hands-free law was enacted, compared to states without a hands-free law. The findings indicate that mobile phone ownership is associated with higher traffic fatality rates in bad weather, on wet roads, and in rush-hour traffic. California’s new law should lead to some 300 fewer traffic fatalities a year. However, to gain the full benefit of the law, California should concentrate its enforcement efforts during adverse driving conditions, and public education about the law ─ and about the distraction and danger of using a mobile phone in the first place ─ should be an important component in implementing the law, which could help change behavior that enforcement alone might not achieve.