We examined the relationship between patients' pretreatment expectations for nausea and vomiting and their subsequent development in a homogeneous group of 29 female cancer patients receiving platinum-containing chemotherapy as inpatients (Study 1) and in 81 subjects with any of a variety of cancer diagnoses treated largely as outpatients (Study 2). Each study found a significant relationship between patients' expectations for nausea development measured prior to their first treatment and their mean postchemotherapy nausea severity (both, p < 0.05). Patients' expectations accounted for unique variance in nausea severity in each study even after controlling for known pharmacological and physiological predictors of nausea (Study 1: delta R2 = .18, p < .04; Study 2: delta R2 = .05, p < .03). By contrast, we found no significant relationships between expectations for vomiting and subsequent vomiting. Our results support the view that patients' expectations for nausea affect its subsequent development, indicating the presence of a significant psychological component in treatment-related nausea. Implications of this are discussed.