Background and study aim: The current study aims to figure out the use of the serological 1, 3 Beta-D-Glucan (BDG) test for early detection of Pneumocystis jirovecii Pneumonia (PcP) among immuno-compromised patients with acute respiratory distress syndrome (ARDS). Patients and methods: This are a cross-sectional study. It was carried out at Ain Shams University Hospitals on immunocompromised patients presenting with pneumonia. The study included 44 clinically relevant patients, with lower respiratory tract infections, based on their clinical presentation and chest radiological findings. The group included 26 males and 18 females. Their ages ranged from 1 to 74 years. Results: Out of 44 immunocompromised Patients with ARDS caused by Invasive Fungal Infections (IFI) suspected PcP according to clinical, radiological findings and Galactomannan, those patients included patients in ICU, with haematological malignancies, on immunosuppressive drugs and HIV positive Patients. 72.7 % were positive for BDG, while 27.3 % were negative for BDG. Positive predictive value of BDG was 75 % while BDG's negative predictive value 91.7 %. Our results figured out also that there was a statistically significant relation between results of BDG and Galactomannan with Value < 0.001, lymphocytic count with P Value 0.005 and LDH level with P Value 0.017. Sensitivity of the test was 96% while Specificity was 57.9% Conclusions: Pneumocystis jirovecii pneumonia represents a huge burden to immunocompromised patients in the intensive care unit (ICU) and those with haematological malignancies, particularly those with lymphoproliferative diseases. Increased rates of ICU admissions for respiratory failure and fatalities are linked to the disease. BDG is a non-invasive diagnostic technique that has a high negative predictive value and can rule out IFI. However, neither do its serum levels accurately reflect the severity of the illness nor are they useful for gauging treatment effectiveness.