Intoxicated patients are not uncommonly admitted to the emergency room (ER) due to unconsciousness. In Japan, 'illegal drug' abuse has increased recently among young people. Diagnosis of 'illegal drug' intoxication is often limited to information from patients or surroundings. The purpose of this study was to evaluate the efficacy of HPLC for the diagnosis of 'illegal drug' intoxication. The clinical records from six patients with 'illegal drug' intoxication who were admitted to our ER between 2000 and 2004 were reviewed. The HPLC system was performed and analyzed using the reversed-phase isolated method. Results are presented in Table 1. HPLC showed the spectrum of drugs from the patients' samples. The chemical component of Rush and Sex Hyper is the same, but they showed different peak retention times. Magic Mushrooms made the varied peaks. Pure White has two chemical components and the two peaks were recognized independently. Pinky made two different peaks in urine. HPLC could be useful in the diagnosis of 'illegal drug' intoxication. However, HPLC has limitations. First, a control drug or a control spectrum is necessary to compare with the spectrum of the patient's sample. Second, HPLC seems to be influenced by additional materials even if they have the same chemical component. Third, we should choose a suitable extract method and column. Because we used acetnitrile not alcohol to extract Magic Mushrooms, HPLC showed many peaks. Finally, urine is not adequate for 'illegal drug' analysis. 2C-I in urine showed different peaks. Serum or gastric juice, before the chemical component is metabolized, should be used.