Background: Mass shootings are common in the United States. They are the most visible form of firearm violence. Their effect on personal decisions to purchase firearms is not well-understood. Objective: To determine changes in handgun acquisition patterns after the mass shootings in Newtown, Connecticut, in 2012 and San Bernardino, California, in 2015. Design: Time-series analysis using seasonal autoregressive integrated moving-average (SARIMA) models. Setting: California. Population: Adults who acquired handguns between 2007 and 2016. Measurements: Excess handgun acquisitions (defined as the difference between actual and expected acquisitions) in the 6-week and 12-week periods after each shooting, overall and within subgroups of acquirers. Results: In the 6 weeks after the Newtown and San Bernardino shootings, there were 25 705 (95% prediction interval, 17 411 to 32 788) and 27 413 (prediction interval, 15 188 to 37 734) excess acquisitions, respectively, representing increases of 53% (95% CI, 30% to 80%) and 41% (CI, 19% to 68%) over expected volume. Large increases in acquisitions occurred among white and Hispanic persons, but not among black persons, and among persons with no record of having previously acquired a handgun. After the San Bernardino shootings, acquisition rates increased by 85% among residents of that city and adjacent neighborhoods, compared with 35% elsewhere in California. Limitations: The data relate to handguns in 1 state. The statistical analysis cannot establish causality. Conclusion: Large increases in handgun acquisitions occurred after these 2 mass shootings. The spikes were short-lived and accounted for less than 10% of annual handgun acquisitions statewide. Further research should examine whether repeated shocks of this kind lead to substantial increases in the prevalence of firearm ownership.