Experimental data obtained in bulk and silicon on insulator (SOI) structures by transmission electron microscopy (TEM) are reported showing that the density of extended defects in SOI structures is reduced in comparison with bulk silicon. Additional data obtained on strained SOI structures show that a less pronounced reduction is observed in these structures. It will also be shown that simulations based on an already existing model and taking into account the effect of the Si/BOX interface acting as a sink for interstitials are not able to explain the experimentally observed defect density reduction in unstrained SOI. (C) 2008 American Vacuum Society.