Background To our knowledge tattooing has never been thought of as a method of introducing nanoparticles (NPs) into the human body by the intradermal route, and as such it has never been a topic of research in nanotoxicology. The content of NPs in tattoo inks is unknown. Objectives To classify the particle sizes in tattoo inks in general usage. Methods The particle size was measured by laser diffraction, electron microscopy and X-ray diffraction. Results The size of the pigments could be divided into three main classes. The black pigments were the smallest, the white pigments the largest and the coloured pigments had a size in between the two. The vast majority of the tested tattoo inks contained significant amounts of NPs except for the white pigments. The black pigments were almost pure NPs, i.e. particles with at least one dimension <100 nm. Conclusions The finding of NPs in tattoo inks in general usage is new and may contribute to the understanding of tattoo ink kinetics. How the body responds to NP tattoo pigments should be examined further.