The increasing age and co-morbidity of dialysis patients may be associated with an increase in the prevalence of Alzheimer's disease, stroke, and other causes of mental impairment. We sought to determine the prevalence, recognition, and implications of mental impairment among chronic hemodialysis patients. We administered the Mini Mental Status Exam (MMSE) to 336 randomly selected patients from three dialysis units. To determine recognition of mental impairment by health care providers, we compared MMSE scores with mental status assessments obtained from each patient's dialysis technician and medical record. To determine the clinical implications of mental impairment, we prospectively obtained Kt/V, albumin, protein catabolic rate, blood pressure, and hematocrit values. To determine the resource implications of mental impairment, we assessed staff time required to care for each patient as well as hospitalizations. We found that 22% of subjects had mild mental impairment (MMSE 18 to 23) and that 8% had moderate-severe mental impairment (MMSE 0 to 17). The sensitivity of technician and medical record mental status assessments were 57% and 15%, respectively. After adjusting for demographic and medical variables, low MMSE score was independently associated with low protein catabolic rate (odds ratio, 1.5; P = 0.02), increased technician time caring for patient after dialysis (odds ratio, 1.5; P = 0.005), and increased hospital days (odds ratio, 1.4; P = 0.03). In conclusion, there is a high prevalence of unrecognized mental impairment among hemodialysis patients that has adverse implications for protein nutritional status, staff time, and hospitalization. We recommend that clinicians routinely screen for mental impairment and target impaired patients for interventions to improve mental status and associated adverse outcomes.