Abstract. Several recent studies suggest that acceleration of the head at impact during sporting activities may have a detrimental effect on cognitive function. Reducing acceleration of impact in these sports could reduce neurologic sequelae. Objective: To measure the effectiveness of a regulation football helmet to reduce acceleration of impact for both low- and moderate-force impacts. Methods: An experimental paired study design was used. Male volunteers between 16 and 30 years of age headed soccer balls traveling approximately 35 miles per hour bareheaded and with a helmet. An intraoral accelerometer worn inside a plastic mouthpiece measured acceleration of the head. The helmet also had an accelerometer placed inside the padding. For more forceful impacts, cadaver heads, both with and without helmets, were instrumented with intraoral (IO) and intracranial (IC) accelerometers and struck with a pendulum device. Simultaneous IO and IC accelerations were measured and compared between helmeted and unhelmeted cadaver heads. The main outcome was mean peak acceleration of the head and/or brain associated with low- and moderate-force impacts with and without protective headgear. Results: Mean peak Gs, measured by the mouthpiece accelerometer, were significantly reduced when the participants heading soccer balls were wearing a helmet (7.7 Gs with vs 19.2 Gs without, p = 0.01). Wearing a helmet also significantly lowered the peak Gs measured intraorally and intracranially in cadavers subjected to moderate-force pendulum impacts: 28.7 Gs with vs 62.6 Gs without, p < 0.001; and 56.4 Gs with vs 81.6 Gs without, p < 0.001, respectively. Conclusions: A regulation football helmet substantially reduced the peak Gs associated with “heading” a soccer ball traveling at moderately high velocities. A helmet was also effective in reducing the peak acceleration both intraorally and intracranially for impacts significantly more forceful than heading a soccer ball.