To estimate the incidence rates and related determinants of head injuries before and after the implementation of a new universal helmet law in Italy. The investigation took place in the Emergency Room of the Accident and Emergency Department, Teaching Hospital 'Umberto I', Rome, in 1999 (before the new law), and 2000 (two periods after the new law). Personal data, injury circumstances, helmet use, others involved in injury and health data, were collected. Incidence rates by time period were calculated and differences between groups were analysed. Logistic regression models were conducted to assess the association between head trauma and potential determinants. The incidence rate of all injury among scooter riders rose from 64.36/10,000 person-years before the new law (1999) to 98.05/10,000 person years afterwards. The incidence rate of head trauma among scooter riders showed an opposite trend, i.e. a decrease from 26.65/10,000 person-years (1999) to 8.88/10,000 person-years in the second post-legislation period. Helmet use among injured scooter riders rose from 5% before the new law to 90% afterwards. Helmet use is a protective factor for being a patient with head trauma, in all the periods considered (OR 0.24-0.44). After implementation of the new law, age (18+) showed a protective effect as well (OR 0.42-0.44). Helmet use has a protective effect on head trauma among scooter riders. One year after implementing a universal law, helmet use has increased substantially and a sharp reduction in head trauma among persons older than 18 years could be observed.