Functional analysis was used to determine the possible function of inappropriate spitting behavior of an adult woman who had been diagnosed with profound mental retardation. Results of an initial descriptive assessment indicated a possible attention function and led to an attention-based intervention, which was deemed ineffective at reducing the frequency of spitting. A follow-up functional analysis revealed an automatic function and an intervention of medication prescribed for gastro-esophageal reflux markedly reduced the frequency of inappropriate spitting. The implications for using functional analysis as a means of identifying biological events such as medical illness are discussed. (Contains 2 figures.)