We collected influent and effluent samples from four sewage treatment plants (STPs) as well as surface water samples in Han River of Seoul, Korea, in three sampling events representing different flow conditions, i.e., April, June, and August, 2005, and analyzed for eleven pharmaceuticals including acetaminophen, caffeine, carbamazepine, cimetidine, diltiazem, trimethoprim, and five sulfonamide antibiotics, using LC-MS-ESI. Pharmaceuticals of high annual production amount were detected in higher level in STP influents. Levels of pharmaceutical residues in the influents were the highest for acetaminophen (average 27,089 ng/L), followed by caffeine (23,664 ng/L), cimetidine (8045 ng/L), and sulfamethoxazole (523 ng/L). Levels of acetaminophen and caffeine in STP effluents were very low compared to the influent concentrations. However cimetidine was detected in relatively high levels even in STP effluent samples. In effluent samples, cimetidine showed the highest level (5380 ng/L), followed by caffeine (278 ng/L), sulfamethoxazole (193 ng/L), and carbamazepine (111 ng/L). The concentration of cimetidine was also the highest in surface water samples (average 281 ng/L), which is the highest level reported from surface water worldwide to our knowledge. Caffeine (268.7 ng/L), acetaminophen (34.8 ng/L), and sulfamethoxazole (26.9 ng/L) were also detected in relatively high levels. Levels of pharmaceuticals detected in surface water samples upstream STPs were generally very low compared to the downstream samples, suggesting that the STPs potentially be a major source of the test pharmaceuticals into Han River. The hazard quotients (HQs) were calculated for the test pharmaceuticals based on their occurrences in surface water, and no pharmaceutical resulted in HQ greater than one, suggesting that their potential environmental impact may be low.