The State of Texas implemented a mandatory total motorcycle helmet law for all operators and passengers, effective September 1, 1989. In this study the impact of this intervention on frequency of both total and head-related fatalities, severe injuries, and overall injuries for operators during the subsequent year was quantified. This quantification is important because 26 states in the United States fail to have strict, mandatory helmet laws. The Box-Tiao time-series intervention methodology is used to estimate secular trends before and changes after the implementation of the law, analyzing Department of Public Safety monthly injury accident data for a period of 6 years collected from traffic accident reports filed for each motorcycle injury accident. Trends in fatalities and injuries (except for head-related deaths) estimated before implementation of the law approximated the 9.4% average annual decline in motorcycle registrations. Additional declines of 12.6% and 57.0%, respectively, were estimated for total and head-related fatalities during the year after the law was implemented. Declines of 13.1% and 54.6% were estimated for severe injuries for total and head-related accidents. Declines of 12.3% and 52.9% were found for total and head-related injuries overall.