To explore the effect of drivers' use of mobile (cell) phones on road safety. A case-crossover study. Perth, Western Australia. 456 drivers aged > or = 17 years who owned or used mobile phones and had been involved in road crashes necessitating hospital attendance between April 2002 and July 2004. Driver's use of mobile phone at estimated time of crash and on trips at the same time of day in the week before the crash. Interviews with drivers in hospital and phone company's records of phone use. Driver's use of a mobile phone up to 10 minutes before a crash was associated with a fourfold increased likelihood of crashing (odds ratio 4.1, 95% confidence interval 2.2 to 7.7, P < 0.001). Risk was raised irrespective of whether or not a hands-free device was used (hands-free: 3.8, 1.8 to 8.0, P < 0.001; hand held: 4.9, 1.6 to 15.5, P = 0.003). Increased risk was similar in men and women and in drivers aged > or = 30 and < 30 years. A third (n = 21) of calls before crashes and on trips during the previous week were reportedly on hand held phones. When drivers use a mobile phone there is an increased likelihood of a crash resulting in injury. Using a hands-free phone is not any safer.