Two trials of external quality assessment for Therapeutic Drug Monitoring (TDM) subcommittee of Korean Association of Quality Assurance for Clinical Pathology (KAQACP) were performed in 2001. Participating laboratories were increased to 80 since the start of TDM proficiency testing in Korea from 1995. Response rates were 89.5% and 93.0% for 1st and 2nd trials, respectively, averaging 91.3%. In the first trial, 19 test items among 26 ones were responded from as least from one laboratory as follows: acetaminophen, amikacin, amitriptyline, carbamazepine, cyclosporine, digoxin, free phenytoin, gentamicin, lithium, methotrexate, phenobarbital, phenytoin, primidone, quinidine, salicylate, theophylline, tobramycin, valproic acid and vancomycin. In the second trial, the test items were same with those of 1st trial except the exclusion of propranolol. The most common test items were digoxin, phenytoin, phenobarbital, carbamazepine, theophylline and valproic acid which were peformed in more than 71% of participating laboratories. The most widely used TDM analyzer was Abbott TDx/TDxFLx (55%), but its proportion were decreased slightly comparing with the previous years. No more laboratory report ethosuximide, and procainamide from 2001. Number of laboratories who reported amikacin, gentamicin, and tobramycin were slightly increased comparing with previous years. In conclusion, we found grossly similar pattern of TDM proficiency testing in 2001 comparing with those of previous years with slightly increase of participation laboratories and slightly decrease of test items.