Nanotechnology has considerable socioeconomic potential. Benefits afforded by engineered nanoparticles (NP: defined as being less than 100 nm in diameter) are expected to be significant in fields such as plastics, energy, electronics, aerospace and medicine. However, NPs are being introduced into the market without adequate assessment of their potential toxicities. It is urgently important to conduct risk assessment of commercial NPs and establish a framework enabling risk management which is not subordinate to their commercial production. An overview of currently available carcinogenicity risk evaluation results of NP materials raises serious questions as to their safety. NP sized titanium dioxide (nTiO(2)) and carbon black (nCB) are carcinogenic to the lung of female rats, and the tumors preferentially include squamous cell morphology. Carbon nanotubes (CNT) induce mesotheliomas when applied intraperitoneally in rats and mice. Data for Fullerenes are insufficient to evaluate carcinogenic risk. Sub-chronic toxicity data indicate that, in general, NPs form aggregates and agglomerates and cause foreign body reactions at their applied sites with inflammatory cell, including macrophage, infiltration. These findings are similar to the biological effects of asbestos, a potent carcinogen, and indicate that careful assessment of NPs is indispensable.