In order to determine whether there was a clinically sufficient amount of drug remaining in used fentanyl patches, quantitative analysis of two different types of patches, each containing 2.5 mg (n=36) and 5 mg (n=20) was performed. After being used for approximately 72 hours by patients with cancer, each patch was put in the plastic bag and stored at $4^{circ}C$ until analysis. Fentanyl remaining in patches was extracted with 50 ml methanol, diluted with water, and counted twice in a $gamma-Counter$ (expressed as CPM). Patches that originally contained 2.5 mg and 5 mg of fentanyl were shown to have $0.48{sim}1.86;mg;(mean:;1.03;mg,;41.16\%);and;0.37{sim}3.95;mg;(mean:;2.37;mg,;47.33\%)$ after use, respectively. A wide interpatient variability was observed in the rate of fentanyl release from patches although the application period was standardized to 72 hours. Since a significant amount of drug remained in the discarded patches, it is highly recommended that patients dispose used ones under supervision to prevent abuse or misuse of the narcotic drug.