Several attempts were carried out to determine the magnitude of nosocomial infection as a health care problem worldwide. This study is a trial to evaluate the nosocomial infection caused by protozoan Giardia lamblia among children admitted to the inpatient clinics of Benha University and BenhaSpecialized Hospitals and to trace the source of infection. 70 children were subjected to the study as well as 18 contacts and healthy workers in addition to 36 water samples .Human infection was investigated by stool examination of 3 successive samples 3 days apart then repeated after 15 days.All samples were examined by direct,iodine staining ,formol ether concentration as well as ELISA coproantigen detection techniques for Giardia lamblia ;while water samples were examined microscopically by direct and filtration techniques as well as ELISA antigen detection for Giardia lamblia parasite .Results out of 70 children , 23 were detected to catch Giardia lamblia from hospital ,they ages 1-5 years , infection rate is higher in immunocompromized cases, cases from rural areas and that from hospital food (nosocomial infection prevalence rate if giardiasis is 22.86%).