The authors studied all tooth fractures identified within a two-week period among adult enrollees in a dental health maintenance organization to determine the incidence and severity of this condition. They found complete fracture rates of 5.0 and 4.4 per 100 adults per year for all teeth and for posterior teeth, respectively, with 15 percent of fractures resulting in pulpal involvement or extraction. These estimates, the first to be reported, provide information that is potentially useful to patients and dentists making decisions about treatment intended to prevent tooth fracture.