The authors evaluated the accuracy of clinical impressions and Mini-Mental State Exam scores for assessing patient capacity to consent to major medical treatment, relative to expert psychiatric assessment. Consecutive medical inpatients (N = 63) facing a decision about major medical treatment received a clinical impression of capacity from their treating physician and the Standardized Mini-Mental State Exam (SMMSE); 48 received independent psychiatric assessment of capacity. Analyses revealed that both clinical impressions and SMMSE scores were generally inaccurate in determining capacity, although all 23 participants with a clinical impression of "definitely capable" were found capable by the psychiatrist. Given the importance of assessing capacity to consent to major medical treatment, better approaches to the clinical assessment of capacity are required. Several strategies are discussed.