The objective of this study was to do an epidemiological and costs analysis of the impact of road traffic injuries on the demand for emergency room services at hospitals located in the city of Cuernavaca, Mexico. The studied population included injured people who demanded medical attention at the emergency room for injuries due to events in public places, occurring between February and April 2001. Trained interviewers collected the data at the emergency room, 24h a day. The statistical analysis included simple and bivariate analysis using chi(2) odds ratio (OR), and a confidence interval of 95%. The costs analysis included the expenses during the process of searching for and finding care for injuries. There were 433 injured people, 233 (54%) were victims of road traffic accidents, 72% of crashes, and 28% were injured pedestrian and cyclists. Variables associated with the demand of emergency care due to road traffic injuries in comparison with other accidents, were: severity of injuries (OR 2.60, CI 1.44-4.71), and thorax injury (OR 4.64, CI 1.03-20.89). Pedestrians had higher costs for health care and 80% of them had to pay out-of-pocket (P<0.05). Differences between patients injured by crashes or as a pedestrian, were: age under 14 years (OR 5.9, CI 2.5-13.9), being unemployed (OR 2.1, CI 1.20-3.96), and being an elementary school student (OR 13.9, CI 3.08-63.13). The present study is, so far, the only one in Mexico to include an epidemiological and costs analysis in approaching the problem of road traffic injuries. Similar methods must be used, especially in developing countries, to reduce this important public health problem.