The ability of cancer cells to become simultaneously resistant to different drugs is a significant impediment to successful chemotherapy. Tc-99m-MIBI has been reported to be a transport substrate for P-glycoprotein (Pgp). The aim of the study was to ascertain the relationship between the degree of Tc-99m-MIBI uptake and the level of Pgp expression in patients with newly diagnosed leukaemia. A total of 26 patients (12 female and 14 male; mean age 46.8+/-3.7 years) with newly diagnosed leukaemia were included in the study. None of the patients had been previously treated with chemotherapy. Images were obtained 20 min post-injection of 740 MBq Tc-99m-MIBI. Whole-body and planar spot images of the pelvis and thorax were acquired. The uptake of the MIBI in the bone marrow was evaluated using a qualitative and also a quantitative scoring system with determination of the tumour-to-background (T/B) ratios. Flow cytometry was performed for determining the Pgp expression of the blast cells in the bone marrow aspiration samples. There was a statistically significant inverse relationship between the Pgp level in numeric values and both mean qualitative (P<0.001; r=-0.665) and quantitative (P=0.001; r=-0.606) results of Tc-99m-MIBI imaging. Both the mean qualitative score and the T/B ratios were higher in patients who were Pgp negative than in those who were Pgp positive (P<0.001 and P<0.001, respectively). These data indicate that an increased level of Pgp expression is correlated with a low accumulation of Tc-99m-MIBI in bone marrow of patients with leukaemia. Tc-99m-MIBI bone marrow imaging, as a method of functional imaging, can give in vivo information concerning the functional expression of the MDR phenotype in patients with untreated leukaemia. ((C) 2003 Lippincott Williams & Wilkins).