This article evaluates the relationship between the pharmacokinetics of methotrexate (MTX), its efficacy in the treatment of rheumatoid arthritis (RA), and serum folic acid (FA) levels. The pharmacokinetics of MTX was studied in 29 patients with RA treated with low-dose MTX. The weekly dose of MTX was given orally at 2-4 mg every 12 h over a period of 24-36 h. Blood samples were taken 4 h after the first administration in any given week. A Bayesian method was used to estimate individual MTX pharmacokinetic variables. We then investigated the efficacy of MTX and the serum FA levels in these patients. We examined C-reactive protein levels (CRP) and the erythrocyte sedimentation rate (ESR), and analyzed the values obtained before and after MTX treatment in order to evaluate the efficacy of the MTX treatment. The degree of improvement in CRP and ESR was significantly correlated with the length of time the MTX concentration-time curve remained above 0.02 microM in one week. Furthermore, the degree of improvement in CRP was also significantly correlated with the area under the concentration-time curve (AUC) for MTX. These results suggest that serum MTX measurements could be useful in determining individual patient regimens.