Polyvinylpyrrolidone superparamagnetic iron oxide nanoparticles (PVP-SPIONs) have unique properties. Due to these characteristics, PVP-SPIONs have been used in several medical applications such as magnetic resonance imaging (MRI) contrast agent or drug delivery system. However, a more comprehensive understanding of the environmental safety of PVP-SPIONs is vital for consumption of these nanomaterials. In this study, we describe the effects of PVP-SPIONs on cell viability of the BT-474 human breast cancer cells. Cell viability of the BT-474 cells treated with PVP-SPIONs (10-800 μg/ml) was assessed by MTT assay. MRC-5 cell line was used as a control. Quantitative real-time PCR was performed to investigate the mRNA expression levels of apoptotic (caspase 3) and anti-apoptotic (BCL2) genes Confluent BT-474 monolayers exposed to PVP-SPIONs showed biphasic effects on cell proliferation. PVP-SPIONs at 10-100 μg /ml promote proliferation of BT-474 cells but not the MRC-5 cells. At higher dosage, PVP-SPIONs have toxicity on BT-474 cells. The results of real-time PCR was in line with MTT assay. The increase of cell proliferation at low PVP-SPIONs concentrations is different from what would be expected for these nanoparticles. Our results suggest that more attentions are needed to ensure the safer use of SPION in nanomedicine.