The uteri of 26 clinically healthy bitches and 42 bitches with a clinical suspicion of pyometra were examined histologically using a computerized image analysis system. Histologic lesions were characterised mainly by thickening or atrophy of the endometrium and by varying degrees of cystic changes of the glands. These lesions were observed in most of the clinically healthy bitches as well as in all of the clinically ill animals. In most of the ill bitches a variable degree of inflammation also was found. Some bitches with clinical signs indicative for pyometra had no inflammatory reaction in the uterus. These bitches were misdiagnosed as suffering from pyometra, confirming the difficulty of diagnosing pyometra by simple clinical examination. Determination of sex hormone serum levels revealed that all dogs in both groups were either in metestrus or in anestrus. Based on the results of this study the cystic endometrial hyperplasia-pyometra complex can be divided in two entities: a cystic endometrial hyperplasia-mucometra complex and an endometritis-pyometra complex. Both entities bear many similarities with each other, except for the inflammatory reaction in the endometritis-pyometra complex. It is concluded from this study that the latter complex probably does not necessarily follow the former, but that both can arise de novo.