The practice of driving while using hand-held mobile phones is diffused in numerous countries, even though it has been demonstrated that they increase the risk of traffic accidents, and such use has consequently been prohibited. Because of the lack of Italian data on the spread of this behaviour a study was done in June 2004 on drivers' use of hand-held mobile phones in the Florence Health Authority area. The study was performed by direct observation from 4 watching-points (1 for each zone) which allowed for observation of several types of vehicles, on varying road conditions, in 5 hour time bands. Overall, 9387 vehicles were observed and the prevalence of mobile phone use while driving was found to be 1.8%. The greatest prevalence of mobile phone use was found in the following conditions: in the Northwest zone (2.8%), in the city suburbs (2.8%), on Tuesdays (3.0%), among drivers not using safety belts (3.0%), and among subjects driving alone (2.1%). Logistic regression analysis has shown a statistically significant association between mobile phone use and the zone, the number of persons in the vehicle, and seat belt use. Mobile phone use while driving is therefore a diffused practice that is particularly common among drivers who engage in other high-risk driving behaviours.