Objective: The purpose of this study was to examine diffusion of and predictors of helmet use among skiers and snowboarders in the Western United States and Canada. Design: 6400 skiers and snowboarders at 29 ski resorts in the Western United States and Canada were interviewed on chair lifts and observed for helmet use during two consecutive ski seasons (winters 2001 and 2002). Setting: Skiers and snowboarders were observed and interviewed at 29 ski resorts in Alaska, California, Colorado, Idaho, Montana, New Mexico, Nevada, Oregon, Utah, and British Columbia as part of a sun protection project. Subjects: Participants completing the survey consisted of 3525 adult skiers and snowboarders in the 2002 season and 2978 adult skiers and snowboarders in the 2001 season. Main outcome measure: The outcome measure for all analyses was prevalence of helmet use by skiers and snowboarders. Results: Helmet use by skiers and snowboarders is increasing and is most prevalent among snowboarders, experts, and more frequent skiers/snowboarders. No evidence was found for the hypothesis that helmet use is diffusing more rapidly among earlier adopters of helmets than later adopters. Conclusions: Although controversy remains, helmets are rapidly diffusing as a safety device at western North American ski resorts. Expert and more frequent skiers and snowboarders are more likely to wear helmets, which may indicate that helmets are recognized as a safety device.