Endometritis is one of the biggest problems affecting horses after colic and lameness. The current study was carried out on 26 mares (4-19 years old) of Straight Egyptian Arabian mares located in different 4 private farms. Ultrasonography revealed that most cases of endometritis were showing intraluminal fluid accumulation, although there was certain cases did not show fluid accumulation yet they suffered from endometritis and subfertility. The degree and severity of fluid accumulation was increased with age of mares. Additionally, there was a relationship between the fluid accumulation and echogenicity and the type of isolated microorganism whereas isolation of Staph and Strept associated with fluid formation and observation of clinical endometritis while in contrast with E.coli, it produced little amount of fluid only high degree of uterine edema was evident. Concerning bacterial isolation there were many incriminated bacteria for example Staphylococcus spp (35.1%), E.coli (18.9%), Proteus vulgaris (13.5%), Streptococcus spp (10.8%), Klebsiella pneumoniae (5.5%), and Pseudomonas aeriogenosa (2.7%) and there was 13.5% bacteriologically negative cases.