An evaluation of the utility of assessing and treating severe problem behavior through precursor functional analysis was completed. Ongoing measurement of problem behavior in two settings in the participant's natural environment was conducted for the duration of the study. A precursor to self-injurious behavior was identified using descriptive assessment and conditional probability analyses. A precursor functional analysis was then conducted. Subsequently, a treatment, in which precursor behavior produced the maintaining variable identified in the precursor functional analysis, was implemented in the natural environment. Treatment resulted in increases in the precursor behavior and decreases in self-injury in both the treatment setting and the second setting in which observations occurred. Implications of the assessment and treatment procedures are discussed.