A simple method for the measurement of the active leflunomide metabolite A77 1726 in human plasma by HPLC is presented. The sample workup was simple, using acetonitrile for protein precipitation. Chromatographic separation of A77 1726 and the internal standard, alpha-phenylcinnamic acid, was achieved using a C(18) column with UV detection at 305 nm. The assay displayed reproducible linearity for A77 1726 with determination coefficients (r2) > 0.997 over the concentration range 0.5-60.0 microg/ml. The reproducibility (%CV) for intra- and inter-day assays of spiked controls was <5%. The limit of quantification was 0.8 microg/ml. The average absolute recovery was approximately 100%. This assay is suitable for the determination of A77 1726 in plasma of patients taking leflunomide, and is simpler to use than other HPLC methods reported previously.