Tungsten (VI) oxide (or tungsten trioxide) (WO3, <100 nm particle size) nanoparticles (NPs) are used for many purposes including production of electrochromic windows, or smart windows, x-ray screen and gas sensors in everyday life. However, their carcinogenicity and genotoxicity have not been sufficiently evaluated. Therefore, the genotoxic potential of WO3 nanoparticle was examined in cultured human lymphocytes by the use of the micronucleus (MN) test and the comet (SCGE) assay. Freshly isolated human lymphocytes were exposed to WO3 nanoparticle at concentrations ranging from 0 to 500 μM for 72 hours at 37°C. Our results indicated that 400 and 500 μM of WO3 nanoparticle treatment caused slight increases of the MN frequencies in cultured human lymphocytes. Likewise, WO3 nanoparticle (at concentrations above 200 μM) led to increases of DNA damage (estimated with the comet assay) in human lymphocytes. The observed alterations in the MN and the comet assay parameters revealed that WO3 nanoparticles have genotoxic potential and could pose environmental and human health risk.