On April 13, 2012, Michigan instituted reforms to its long-standing universal helmet law (UHL) and thus became the 28th state with a partial helmet law, which requires helmet use by only a subset of the riding population. Given continuing increases in motorcycle fatalities, helmet use remains a divisive policy issue facing state governments. The existing research literature includes several before-and-after studies that evaluate the effects of changes in motorcycle helmet laws on metrics such as helmet use and statewide motorcycle fatalities. However, a comprehensive assessment of the effects of helmet use laws on the full range of crash injury outcomes is lacking. Important evidence was added to inform the continuing debate about the efficacy of UHLs. A detailed, disaggregate-level study was conducted to assess the degree of injury severity sustained by motorcyclists involved in crashes before and after Michigan's transition from a UHL to a partial helmet law. By controlling for various rider, roadway, traffic, and weather characteristics, results of the study demonstrate that helmets reduced the probability of fatalities by more than 50%. Injuries tended to be less severe in crashes that involved deer, occurred at lower speeds, or occurred in inclement weather but more severe in high-speed collisions or when drugs and alcohol were involved. The riders who tended to be more susceptible to severe injury were female, younger (age 21 to 30 years), or older (age 51 to 70 years).