2,944 arrestees urine specimens submitted (during the period of September 2000 to February 2001) by law enforcement agencies in three cities and three counties in Taiwan were tested for methylenedioxymethamphetamine (MDMA) and delta-9-tetrahydrocannabinol-9-carboxylic acid (THCA) to monitor the use of MDMA and marijuana among the sampled population. Adapted preliminary tests were fluorescence polarization immunoassay (FPIA, TDx) for amphetamine/methamphetamine (cutoff: 500 ng/mL d-amphetamine) and enzyme immunoassay for marijuana metabolites (cutoff: 50 ng/mL delta-9-tetrahydrocannabinol-9-carboxylic acid, THCA). GC/MS tests cutoffs for MDMA and THCA were 500 and 15 ng/mL, respectively. Among the specimens tested, immunoassay positive rate for amphetamines was 61.7%, while confirmed positive rates for MDMA and THCA were 1.15% and 0.37%, respectively. All positive specimens came from the thee cities, with Taipei City leading the positive rates of 2.49% for MDMA and 0.65% for THCA. The corresponding positive rates for specimens from Taichung City and Kaohsiung City were 1.26%, 0.42% and 0.60%, 0.36%, respectively. No specimen was found positive for both MDMA and THCA. All THCA and the majority of MDMA positive specimens (57%) tested negative for methamphetamine, suggesting that the majority of the marijuana and MDMA abuse population are different from those abusing methamphetamine. Results from this survey also indicate that MDMA and marijuana abuse are concentrated in the metropolitan areas of Taiwan.