Dementia is one of the most serious problems in aging societies. A recent study has reported that higher serum coenzyme Q10 levels could reduce the risk of incident dementia. In this community-based study, we evaluated changes in serum ubiquinol level and cognitive performance following long-term intake of ubiquinol. Sixty-one residents (26 men, 35 women; aged 33-87 years) of Kamijima town in Ehime Prefecture agreed to participate in this study. The participants consumed 100-150 mg ubiquinol per day for 6 months to 2 years. We measured serum ubiquinol level and assessed cognitive performance using the 1-min Digit Symbol Substitution Test (DSST) both at baseline and after long-term ubiquinol intake. We also examined the subjects after a 3-month washout period. At baseline, a strong negative correlation was found between age and DSST score. There was no significant correlation between serum ubiquinol level and DSST score. Following long-term ubiquinol intake, both serum ubiquinol level and DSST score increased significantly. After the 3-month washout period, serum ubiquinol level decreased to almost the baseline level, whereas DSST score did not change significantly. Because this study is a single-arm trial these results should be considered carefully; nonetheless, our findings indicate the probable effects that long-term ubiquinol supplementation improves cognitive performance among community residents.