Cognitive impairment is common in hemodialysis (HD) patients. The mini mental status examination is a simple screening test for dementia. The objectives of this study were to (1) study and compare the predialysis and postdialysis mini mental status examination score and 2 subscores and compare them with those of a control group and (2) determine the factors affecting these scores. This was a prospective study of 54 HD patients, which involved calculation of their predialysis (PrHDSc) and (2-4 weeks later) postdialysis (PoHDSc) scores and comparison of these with the control scores (CoSc). The mean scores for PreHDSc and PoHDSc were 26.5+/-2.7 and 26.4+/-3.3, respectively. Both were significantly lower than CoSc, 28.4+/-1.6 (95% CI for score difference 0.99-2.97, P<0.001). The subscores for orientation, registration, and recall (ORR) and attention (ATT) before and after HD were 14.2+/-1.3, 14.3+/-1.8, and 3.5+/-1.7, 3.2+/-1.8, respectively. Both were significantly lower than the CoSc, 15.2+/-1.2 and 4.2+/-1.1 (P=0.001 and 0.004, respectively). There were no significant differences between the PrHDSc and PoHDSc (P values of 0.87, 0.63, and 0.45, respectively). Patients' PrHDSc correlated positively with PoHDSc and dialysis efficiency measured by the urea reduction ratio and Kt/V (r=0.58, 0.4, and 0.34, respectively). Education level correlated positively with PrHDSc r=0.41 but not PoHDSc. Hemodialysis duration correlated negatively with PrHDSc r=-0.3. There was no correlation among age, chronic renal failure duration, HD frequency, weight loss, systolic or diastolic blood pressure drop, and PrHDSc or PoHDSc. Hemodialysis patients scored significantly less than the control patients. Their score was not affected by HD. This may reflect the stable cognitive function/dysfunction or the mild sensitivity of the test.