Pentavalent 99mTc-dimercaptosuccinic acid [99mTc-(V)DMSA or (V)DMSA] is a marker of phosphate transport, entering cells specifically through type III NaPi co-transporters. Phosphate ion is known to be involved in cell metabolism, including the apoptotic cell death process. As phosphate accumulation decreases during apoptosis, we investigated the influence of type III NaPi co-transporter activity on (V)DMSA uptake during this type of cell death.Uptake of (V)DMSA and phosphate was compared in a leukaemic cell line (U937) in vitro model after induction of apoptosis by a chemotherapeutic agent, etoposide (VP16). (V)DMSA biodistribution in nude mice during apoptosis was also investigated in a U937 xenograft in vivo model. The percentage of apoptosis in vitro and ex vivo was determined with annexin V fluorescein by flow cytometry.The in vitro results showed that, in parallel with the decrease in phosphate uptake during apoptosis, (V)DMSA accumulation is negatively correlated with the percentage of apoptosis. Biodistribution studies showed decreased accumulation of (V)DMSA in tumours after treatment with VP16. Animal studies also confirmed an inverse correlation between percentage of apoptosis in tumours and (V)DMSA uptake.The activity of type III NaPi co-transporter is inhibited during the early stages of apoptosis, leading to differential incorporation of (V)DMSA in viable cells and apoptotic cells both in vitro and in vivo.