To optimize the neutron dose for boron neutron capture therapy (BNCT), the boron-10 (10B) concentration kinetics of 10B-p-boronophenylalanine (BPA) were analysed in 22 melanoma patients with primary or metastatic melanomas who received BPA and subsequently underwent BNCT or surgery. The blood concentration in nine patients receiving 179.7+/-14.9 mg/kg BPA increased with time during intravenous infusion, peaked at the end of administration and decreased thereafter. The peak values at the end of administration were 9.4 2.6 microg 10B/g blood, and half-lives for the initial and second components of the blood clearance were 2.8 and 9.2 h, respectively. Skin concentrations in the 10 patients varied from case to case; however, skin-to-blood ratios were relatively constant at 1.31+/-0.22 during the 6 h after the end of administration. Boron concentrations in the tumours resected from the seven patients who were operated on decreased in parallel to the blood values, the tumour-to-blood ratio being relatively constant at 3.40+/-0.83. The present analytical data of BPA pharmacokinetics support our previous approach for optimizing the timing of irradiation and setting the neutron flux large enough for tumour eradication but still tolerable for normal skin.