The adulteration by synthetic therapeutic substances of traditional Chinese medicines has been reported on various occasions and has been a public health concern in Taiwan over the past several years. A large-scale effort was initiated in 1992 to screen traditional Chinese medicines that were suspected of adulteration with synthetic therapeutic substances. The term "adulteration" refers to traditional Chinese medicines that are tested and found to contain chemical substances not prescribed or labeled as part of the intended use. A total of 2,609 samples were collected by eight major general hospitals in Taiwan. Samples were collected through physicians' referrals during patients visits. The samples were analyzed by hospital pharmacists following the established standard procedures in comparison to references by thin-layer chromatography. An average of 23.7% (n = 618) of the samples collected from the eight hospitals were adulterated. Four samples with either a rheumatoid or an antiinflammatory indication contained six different kinds of adulterants. More than half (52.8%) of the adulterated traditional Chinese medicines contained two or more adulterants. The sources of adulterated samples and their claimed indications, as well as the most frequently detected synthetic therapeutic substances, are presented in this report. The controversies regarding the combination of synthetic therapeutic substances and traditional Chinese medicines without adequate labeling should be resolved through regulatory actions for better safety of drug use.