HA-coatings have been widely used in humans surgery to increase the biocompatibility of metal alloys. Their characteristics (i.e. composition, crystallinity, porosity) are of fundamental importance for their behaviour in the organism and in particular their degradation. We had the opportunity to perform histological analysis on human femurs containing HA-coated hip prosthesis implanted for various periods from a few days up to several years. It was showed that the coatings degraded after a few weeks and the released debris get different fates according to their size and shape. The results observed in humans were compared with those obtained in animals or in vitro and some general rules can be drawn regarding the coating characteristics necessary for an optimised biocompatibility.