A number of prognostic factors have been reported to influence the probability of developing nausea and vomiting after cytotoxic chemotherapy. This study used data collected in four randomized anti-emetic trials conducted by the Clinical Trials Group of the National Cancer Institute of Canada (NCIC-CTG) to assess the consistency of the effects of these prognostic factors. A total of 582 patients, all of whom had received moderately emetogenic chemotherapy for the first time, but who were assigned to different anti-emetics, were included in the analysis. The major findings was that the probability of post-chemotherapy nausea and vomiting was much more strongly influenced by the type of chemotherapy given and the type of anti-emetic used than by patient (e.g., age, gender) or environmental (e.g., treatment location, time of administration) characteristics. Further, patient-related factors had different, and sometimes opposite, effects in different anti-emetic and chemotherapy subgroups. Finally, the relative potency of anti-emetics appeared to vary with chemotherapy regimens. Implications of these findings for future studies are discussed.