While there have been considerable efforts to promote bicycle helmet use in the United States in recent years, little information has been available on actual helmet usage patterns. Based on the results of a recent national survey, this article provides information on current helmet usage patterns, the types of helmets in use, and the reasons why bicycle riders use or do not use helmets. It also presents a statistical analysis of the factors associated with helmet use. These factors are determined and quantified with a probit regression model, a type of qualitative response model that can be used to estimate the expected probabilities of helmet use for individual bicyclists and for various population subgroups. The regression analysis shows that helmet use is systematically related to riding patterns, rider characteristics, and household demographics.