In 1992 a new Traffic Rules Code was enacted in Spain. This Code included a helmet law that came into force on 1 September 1992. Since then, helmet use is compulsory for motorcycles in urban areas. Previously, helmet use had been obligatory only for motorcycle in highways. To evaluate driver and passenger compliance with the helmet law, an observational prevalence survey of helmet use in Pamplona (a city of 183,539 inhabitants in the north of Spain) was performed. The utilization of helmet by motorcycle drivers and their passengers was recorded three months before and three after the helmet law took effect. Helmet use increased from 19.7% in the first period to 94.8% in the second period. After the law took effect helmet use was higher among drivers 97.5% than among passengers 77.5%. Passengers used helmet more frequently when the driver was also using helmet. The prevalence ratio of helmet use, in those passengers whose driver used helmet versus those who didn't, was 9.91 (95% CI = 1.52, 64.83). Health education and law enforcement efforts should be targeted to drivers to prevent them from giving a ride to passengers without helmet.