The spatial distribution of X-ray synthesized, PEG coated Au nanoparticles in cancer-bearing mice and their time dependent accumulation were investigated with inductive coupled plasma - optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM) and histological imaging. The results conclusively demonstrate that the particles strongly accumulate in tumor regions; up to similar to 25 times more than in normal muscle tissue. This accumulation increases with the time after injection for up to similar to 12 h in tumor, spleen and liver tissues, whereas for most non-tumor regions it saturates or decreases (blood, lung, brain, heart, and kidney). The impact of this result is discussed with special emphasis on passive targeted drug delivery and could also be used for the delineation and early-stage imaging of small tumors. (C) 2010 Elsevier B.V. All rights reserved.