Therapeutic drug monitoring (TDM) services for plasma concentrations of haloperidol and bromperidol using enzyme immunoassay (EIA) methods are available in Japan, whereas high-performance liquid chromatographic (HPLC) methods are preferred in other countries. To compare these methods, we took 54 plasma samples for haloperidol and 91 plasma samples for bromperidol from schizophrenic patients receiving haloperidol or bromperidol, and the samples were measured using both commercial EIA and HPLC methods. Significant linear correlations were found between the two methods in determining haloperidol (EIA = 1.351 x HPLC + 1.39; r = 0.934, P < 0.001) and bromperidol (EIA = 1.420 x HPLC + 0.712; r = 0.956, P < 0.001) concentrations, but plasma concentrations using the EIA kits were approximately 92% (95% CI; 53-131%) and 62% (54-70%) higher than those using HPLC for haloperidol and bromperidol, respectively. Mean (and range) plasma concentrations of reduced metabolites were 54% (30-92%) and 55% (29-111%) of those of haloperidol and bromperidol, respectively. The present study suggests that reduced metabolites are included to a considerable degree in TDM data using the EIA kits. Therefore, some limitation of TDM data of haloperidol and bromperidol using the EIA kits, ie, high precision but poor accuracy, should be kept in mind.