5-HT₃ receptor antagonists are widely used for prevention of chemotherapy-induced nausea and vomiting, though their antiemetic effects vary among patients. We investigated a method for evaluation of antiemetic effects in individual patients. We used the 5-HT₃ receptor occupancy of serotonin for our evaluation, which was estimated based on the plasma concentration of granisetron and concentration of serotonin near the 5-HT₃ receptor in the small intestine, obtained by measuring the urinary concentrations of granisetron and 5-hydroxyindoleacetic acid (5-HIAA)/creatinine (Cre). The mean cumulative percent for urinary excretion of granisetron at 24 h after administration and coefficient of variation were 16.19±6.30% and 38.91%, respectively. The time course of urinary concentration of 5-HIAA/Cre also varied among the patients. The value for 5-HT₃ receptor occupancy of serotonin without granisetron was higher than that prior to administration (blank), thus most treated patients had the possibility of induced emesis. In contrast, that with granisetron was lower than the blank value, indicating that those treated patients would not develop emesis. Furthermore, the estimated 5-HT₃ receptor occupancy of serotonin in the small intestine and actual individual patient condition corresponded well, showing the validity of our method. Our results suggest that it is possible to evaluate individual antiemetic effects by estimating the 5-HT₃ receptor occupancy of serotonin in the small intestine based on plasma concentrations of granisetron and serotonin near the 5-HT₃ receptor in the small intestine using noninvasive urine samples. This method of individual evaluation is considered to be useful and effective.