This study investigated the temporal distribution of risky behaviors among injured motorcyclists, that is, riding unhelmeted or while intoxicated, and showed how they are associated with risk of fatal injuries. Data of motorcyclists injured in Khon Kaen municipality in northeastern Thailand and transferred to Khon Kaen Regional Hospital were obtained from the trauma registry system of the hospital. Case fatalities were compared by time of day, age group, helmet use, and alcohol intoxication. Unhelmeted riding peaked late in the evening and riding while intoxicated peaked around midnight. Both were associated with increased fatality risk after stratification by time of day; the odds ratios were 3.49 (95% confidence interval (CI) = 1.48-9.36) and 3.01 (CI = 1.71-5.19), respectively. Nighttime injuries were not significantly associated with increased fatality risk after stratification by helmet use or alcohol intoxication. Unhelmeted driving was prevalent and associated with higher fatality risk among younger drivers, whereas intoxicated driving was less prevalent among teens but associated with increased risk among those aged 20-39 years. This study shows that riding unhelmeted or while intoxicated can explain the increased fatality risk at night, suggesting that safety education or enforcements should be targeted at specific age groups and appropriate times.