A sensitive (200 ng/g) and selective reversed-phase high-performance liquid chromatography separation has been developed to determine the levels of tamoxifen, 4-hydroxytamoxifen (4-OH) and desmethyltamoxifen (DMT) in tumour tissue taken from patients undergoing tamoxifen therapy. A μBondapak C18 10 μm column (30 cm×3.8 mm I.D.) was used, with a mobile phase of methanol-1% triethylamine at pH 9 (89:11, v/v). Sample preparation was carried out using a C2 (500 mg sorbent, 3 ml reservoirs) solid-phase extraction method, and extraction efficiencies were followed in individual extracts using a [3H]TAM radiolabelled spike (10 000 dpm), with a range of 60–90%. Accuracy and precision (standard deviation) as determined from tumour spiked with radioinert tamoxifen and its metabolites ranged from 83.4–92.3% (±23–33%) at 20 μg/g; 85.2–87.7% (±18–23%) at 2 μg/g; 88–101% (±15–50%) at 0.2 μg/g and 63–94% (±13–24%) at 0.02 μg/g. Results from seventy-two patients show mean values (±S.D.) of 174±203 ng/g for 4-OH; 783±1326 ng/g for DMT and 410±458 ng/g for TAM, variations reflecting heterogeneity in levels between patients. This methodology can be routinely applied to the determination of tamoxifen and its metabolites in tumour tissues from patients undergoing tamoxifen therapy.