Objective: To find the common etiological factors causing pancytopenia in patients presenting to tertiary care hospital in Islamabad Study Design: Cross sectional observational study Place and duration of study: The study was conducted within the department of Hematology (Pathology), Pakistan institute of medical sciences (PIMS) Islamabad from January 2018 to June 2018 Patients and Methods: Total 50 patients were included in this study. Proper history was taken from all patients and a thorough physical examination was also performed and all findings were documented. All base line investigations were done before performing bone marrow aspiration. Investigations were done from the laboratory within the hospital. CBC, serum profile, bleeding and clotting profile were checked before performing procedure. Data of each patient was documented properly. Consent was taken from all patients and also from the in charge of pathology department for conducting study. Data was analyzed using Microsoft office and SPSS version 2012. Data was expressed in the form of tables. Result: 260 patients from different units were received for bone marrow biopsy during 6 months duration. 50 patients out of 260(19%) were having pancytopenia. The most common cause of pancytopenia was found to be megaloblastic anemia (18%), followed by hypersplenism (16%) and infection related changes (16%). Megaloblastic anemia was most commonly found in the middle aged population (>40-60 years) Conclusion: The study showed that single most common cause of pancytopenia in adult population is megaloblastic anemia, followed by hypersplenism and overwhelming infection. Bone marrow examination is most useful investigation in understanding the etiology of pancytopenia.