The effect of toremifene on P-glycoprotein-mediated multidrug resistance (MDR) in breast and head and neck cancer cell lines was measured in vitro and in vivo. Pgp expression was low and high, respectively, in drug-sensitive (MCF7-S, KB) and drug-resistant (MCF7-R, MCF7-R1, KBV1) cell lines. Toremifene (7.5 μM) significantly enhanced cytoplasmic and nuclear accumulation of doxorubicin in drug-resistant cells. Toremifene (10 μM) increased the in vitro cytotoxicity of doxorubicin in drug-resistant breast cancer cells (13-fold and 21-fold for MCF7-R and MCF7-R1, respectively) without affecting the sensitivity of MCF7-S cells. Similarly, toremifene (10 μM) caused a 12-fold increase in the sensitivity of KBV1 cells to vinblastine. In contrast, toremifene (5 μM) reduced the net uptake of the radiolabelled Pgp substrate, Tc-99m-sestamibi, in the Pgp-overexpressing cell lines by factors of 0.32 and 0.42 for MCF7-R1 and KBV1 cells, respectively (p < 0.01), and, to a lesser extent, by corresponding factors of 0.89 and 0.86 in the drug-sensitive cell lines (p < 0.05 and p > 0.05, respectively). In nude mice bearing both KB and KBV1 xenograft tumours, significantly higher tumour levels of Tc-99m-sestamibi were recorded in KB tumours compared with KBV1 tumours. After 3 days of treatment with intraperitoneal toremifene (25 mg/kg), tumour levels of Tc-99m-sestamibi were reduced in KB and KBV1 tumours but only statistically significantly for KB tumours. Toremifene is a potent MDR modulating agent with respect to chemotherapeutic agents but has the opposite effect with respect to Tc-99m-sestamibi. This finding is of importance in view of the widespread use of Tc-99m-sestamibi as an imaging surrogate for a chemotherapeutic agent.