Acarbose is an antidiabetic drug that inhibits alpha-D-glucosidase and alpha-amylase. We have found discrepancies of serum and urine amylase activities (AA) determined with different assay methods using samples collected from diabetes mellitus patients taking acarbose. As a result of our screening test using the urine samples, we found a 22% probability of discrepancy (the differences between the two methods were > or = 10% at AA > or = 100 U/l). We have measured acarbose in five discrepant samples by high-performance liquid chromatography/mass-spectrometry and quantified very low levels (0.36-0.93 micromol/l) of acarbose in three samples of urine. The inhibition of AA was known not only to be caused by acarbose but also its metabolites. We should suppose that the inhibitory effects of its metabolites on amylase measurements are larger than those of acarbose.