The migraine prophylactic herb feverfew (Tanacetum parthenium L.) is marketed in the United States in a variety of forms and compositions. Although its therapeutic efficacy is still uncertain, the sesquiterpene lactone parthenolide is the constituent recommended to be measured for quality control of feverfew preparations. A validated liquid chromatographic method was developed and used to estimate parthenolide in a number of U.S. feverfew market products formulated as capsules, tablets, or crude powder. The method uses a Lichrosphere 5 C18 column, a mobile phase consisting of 50 mM NaH2PO4 in H2O (solvent A), and CH3CN-MeOH (90 + 10, v/v; solvent B). Elution was run at a flow rate of 1.0 mL/min with a linear gradient of 50-15% A in B over 20 min and UV detection at 210 nm. The correlation coefficient for the calibration curve was 0.9999 over the range of 0.00-0.400 mg/mL. Overall recovery of parthenolide was 103.1%.