The pharmacokinetics of orally administered tacrolimus were examined in six female lupus nephritis patients (mean age 43 years, range 24-55 years). Tacrolimus (3 mg) was administered after supper, and blood tacrolimus concentrations were measured just prior to dosing and 1, 2, 4, 6, 8, 12 and 24 h after administration. The maximum blood concentration (C_max) was observed 4-8 h (mean: 6.7 h) after administration. The mean C_max and area under the tacrolimus concentrationti-me curve (AUC_{0-24 h}) were 12.7 ng/ml and 163.1 ng·h/ml, respectively. Although there was a weak correlation between AUC_{0-24 h} values and tacrolimus concentrations 2, 4, and 6 h after administration, concentrations at 12 h and 24 h were highly correlated with AUC_{0-24 h} values, suggesting that the trough concentration (C_{24 h}) and C_{12 h} are valid markers for therapeutic tacrolimus monitoring. Enzyme-linked immunoabsorbent assay (ELISA) and microparticle enzyme immunoassay (MEIA) measurements of blood tacrolimus concentrations were similar. We recommend that monitoring should be carried out by C_{12 h} in lupus nephritis outpatients.