Explanations of individual differences in susceptibility to learning of anticipatory nausea and/or vomiting (AN/V) in cancer patients have focused mainly on the patients who develop AN/V. On the basis of conditioning theory, however, one would expect that all patients who experience post-treatment nausea and/or vomiting (PN/V) should develop AN/V. Consistent with findings demonstrating that conditioned responses are more easily established in autonomic-reactive individuals, we have previously reported that patients who do not develop AN/V (noAN/V) are less autonomic reactive than the AN/V patients. Thus, we hypothesized that the noAN/V patients might not show classical conditioning to the same degree and extent as the AN/V patients and that conditioning may be the mediating mechanism in AN/V. This study presents data from 36 patients. Among the 28 patients who experienced PN/V, 16 developed AN/V. We found no systematic group differences in severity of PN/V or antiemetic treatment. Before chemotherapy, all patients were tested in a signaled reaction-time paradigm. We used two different tones (CSs), one of which was always followed by a noise (UCS) as a button-press signal. The patients who later developed AN/V demonstrated significantly shorter reaction times, and also showed enhanced cardiovascular reactivity to the tone followed by the UCS (CS+) in comparison with the nonsignal tone (CS-). The noAN/V patients did not show differential cardiovascular responses to the CS+ and CS-.