Atomic force microscopy (AFM) has been systematically used to image the surface of a femoral head made of alumina/zirconia composite (henceforth referred to as zirconia-toughened alumina, ZTA), in comparison with two types of commercially available femoral heads made of monolithic zirconia. AFM experiments were conducted before and after in vitro exposure in water moist environment. All materials contained zirconia partly stabilized with yttria. AFM observations were performed on wide areas of several heads of each type (i.e., in the order of several tens of μm2), in order to ensure statistical reliability in the topologic measurements. Tetragonal-to-monoclinic phase transformation, which was quantitatively characterized by confocal Raman spectroscopy, showed significant difference among the investigated samples. Such differences were similarly found with respect to both roughness and time needed for topologic changes to occur. Variation of topologic statistic parameters, such as skewness and kurtosis of surface height histograms, confirmed the main impact of zirconia grain size on the environmental stability of the head surface, the finer the grain size the higher the stability.