Background and study aim: Fever of unknown origin (FUO) is a state of febrile illness for more than three weeks, with a body temperature greater than 38.3 °C on several occasions and uncertain diagnosis. Conditions blamed for FUO were categorized as infectious diseases, non- infectious inflammatory diseases, neoplasms along with miscellaneous causes. This study aims to identify the infectious causes of FUO counting the parasitic causes in Port-Said Fever Hospital/ Egypt. Patients and Methods: 341 patients admitted to Port-Said Fever Hospital/ Egypt during the period from November 2017 till May 2018 complaining of prolonged fever ≥ 3 weeks, without a definite diagnosis, were included in this study. Complete history taking and clinical data including pattern, duration and degree of temperature, in addition to other co-morbidities, radiological examination, laboratory investigations, and clinical evaluation were reported for each patient. Results: The majority of affected cases were adult males, lived in urban areas, and were in contact with animals. Luncheon and beef were the commonest types of consumed food among the studied population. The uppermost detected infection was tuberculosis (17%) followed by typhoid and Brucella infection (11.7% for each), toxoplasmosis and hepatitis were detected in 2.9% and 1.8% respectively. After investigations, 5.9% of the studied group had hepato- splenomegaly and 77.4% of them presented with lymphocytosis. Clinically, 78.9% of patients had lymphadenopathy and 1.8% had jaundice. Additionally, 75.4% of cases presented with an intermittent fever while 24.6% with continuous fever. Approximately 57.2% of them had fever for three weeks and the majority of patients had fever ≥ 39°c. Conclusion: Tuberculosis, typhoid fever, and brucellosis signified the major identified infectious causes for FUO in Port Said governorate/Egypt. Toxoplasmosis can be represented as one of FUO infectious causes. Patients with toxoplasmosis either presented with lymphadenopathy or not, all of them had hepatosplenomegaly, high CRP, leukocytosis and eosinophilia.