5-hydroxy-3-methylindole was first found by Spatz, Spatz, and Fischer in the urine of patients with centrencephalic epilepsy in 1972. I have determined this substance in human urine by gas chromatography with an electron capture detector (ECD). One hundred ml of urine was filtered, concentrated in vacuo and extracted by benzeneethanol(9.5: 0.5 v/v). The extract was passed through silica gel column. The eluate was dried completely in vacuo, dissolved in acetonitrile, trifluoroacetylated by trifluoroacetic acid anhydride and then determined by gas chromatography (ECD). The detected peak in gas chromatogram has been confirmed as di-trifluoroacetylated 5-hydroxy3-methylindole by GC-MS. As the results of the experiment, it was found that about 5.5-90.0 μg/day of HMI was excreted in normal subjects and extraordinary high level(108.0-613.2 μg/day) was observed in epileptics.