Abstract: Nanotoxicology evaluates the relationship between the structure properties of nanoparticles and toxic hazard, which is of considerable importance prior to clinical application of nanosystems. In spite of the introduction of nanotoxicology in regulatory affairs and the elucidation of some physicochemical structure – toxicity relationships by means of in vitro and in vivo research of nanomedicines, the acquired knowledge still does not allow the prediction of chronic toxicity, especially subtle ways of cell function impairment such as carcinogenesis in humans. This review focuses on potential nanoparticle toxicity after dermal or inhalation exposure. Special emphasis is given to intravenous administration of nanoparticles and the related hematotoxicity. Here, we discuss the main factors, affecting the toxicity of nanoparticles for medical application, namely size, surface charge, chemical composition and the interaction with biological matrices.