Current theorizing on classical conditioning has emphasized the role of contingent relations between the conditioned and unconditioned stimuli in the development of conditioned responses. The present study is the first to examine the relevance of this concept to our understanding of the phenomenon of anticipatory nausea in cancer chemotherapy patients. Anticipatory nausea in patients receiving emetogenic chemotherapy has been cited as an example of the importance of classical conditioning in clinical medicine. Outpatient chemotherapy can be viewed as a series of conditioning trials in which the previously neutral stimuli of the clinic (conditioned stimuli) are associated with chemotherapy infusions and postinfusion nausea. Reexposure to these clinic stimuli alone is sufficient to elicit nausea (conditioned response) in some patients prior to subsequent infusions. In the present study we examined whether differences among patients in percent reinforcement (the percentage of infusions followed by nausea) would predict anticipatory nausea, which was assessed at the sixth infusion. Results were consistent with the hypothesis. Percent reinforcement was positively correlated with the incidence of anticipatory nausea. Comparison of patients with and without anticipatory nausea (t-test and hierarchical logistic regression analysis) confirmed that percent reinforcement was a significant predictor of anticipatory nausea, independent of other factors previously reported to be involved.