The performance of two groups of hospitalized mentally ill patients (schizophrenia and major depression) and two groups of non-mentally-ill patients (patients hospitalized for ischemic heart disease and non-ill primary care patients) was compared on a standardized, objective instrument for assessing patients' understanding of information relevant for patient decision making (consent) about treatment with medication. Generally, hospitalized schizophrenic patients manifested significantly poorer understanding of "informed consent" disclosures about potential medication than did the other groups. Considerable variance, however, was apparent within the schizophrenic group and was related to a number of clinical and demographic variables. The results are interpreted with reference to issues of competence to consent to or refuse treatment.