Tanacetum parthenium (feverfew) is a botanical product that contains parthenolide. Parthenolide has in vitro and in vivo antitumour and anti-angiogenic activity. A Phase I trial was conducted to evaluate the pharmacokinetics and toxicity of parthenolide. Tanacetum parthenium (Tanacet trademark) was administered as a daily oral tablet in a 28-day cycle. A starting dose of 1 mg per day was explored with subsequent dose escalations to 2, 3 and 4 mg. Assessment of plasma pharmacokinetics was performed. Solid-phase extraction and mass spectroscopy were used to evaluate parthenolide plasma concentrations. The limit of detection for parthenolide in plasma was 0.5 ng/ml. Patients were evaluated for response after every two cycles. T. parthenium given on this schedule had no significant toxicity and the maximum tolerated dose was not reached. When parthenolide was administered at doses up to 4 mg as a daily oral capsule in the T. parthenium preparation, there was no detectable concentration in the plasma. Because of this, parthenolide pharmacokinetics were not able to be completed. T. parthenium, therefore, does not provide detectable plasma concentrations. Purification of parthenolide for administration of higher doses will be needed.