Two trials of external quality assessment for Therapeutic Drug Monitoring (TDM) Subcommittee of Korean Association of Quality Assurance for Clinical Laboratory were performed in 2006. The number of participating laboratories were increased to 104, by 9.5% increase comparing with the previous year. Response rates were 100.0% for both trials as the recent 4 years. Two kinds of control materials were requested simultaneously to be tested in each trial so that each institution could know the possible systematic error. In both trials, 20 test items were responded at least from one laboratory. The average drug item was 6.5 per institution, which was decreased slightly from 6.8 in recent 5 years. The most common test items were valproic acid, digoxin, phenytoin, carbamazepine, and theophylline which were peformed in more than 71% of participating laboratories. The response rate of phenobarbital were decreased to 61%, followed by cyclosporine, lithium, vancomycin, tacrolimus, methotrexate, amikacin, gentamycin, salicylate, tobramycin, acetaminophen, salicylate, primidone, free phenytoin, and amitryptyline, in decreasing order. The most widely used TDM analyzer was Abbott TDx/TDxFLx (38.9%), followed by Abbott AxSym (25.9%), and Roche Cobas Integra (20.5%). The inter-laboratory coefficients of variations were not greatly improved comparing with those of previous years. In conclusion, the TDM external quality assessment of 2006 was showed the similar patterns except the increase of the participating laboratories (n=104) with the slightly decreased usage of popular 6 items.