Aim: Investigating the cause of Pyrexia of unknown origin using bone marrow examination. Materials and Method: Total number of 100 patients were selected for this study based on the Petersdorf and Beeson criteria and age above ≥2 years and below ≤70 years. Patients clinical, radiological and laboratory findings were recorded. Preliminary investigations include complete haemogram, peripheral blood smear for malarial parasites, Widal test, urine rutine examination, liver function tests, urea & creatinine, chest x ray and Mantoux test. Bone marrow aspiration (BMA) were performed after taking informed consent from patient and posterior superior iliac spine was the site preferred under local anaesthesia using bone marrow aspiration needle. Results: All the patients had prolonged fever, ranging from 3 weeks to 2 months. There was history of weight loss in 42 cases, history of diarrhoea in 7 cases, epistaxis seen in 12 cases, history of rash and jaundice seen in 7 cases. Hepatosplenomegaly was seen in 45 cases and lymphadenopathy in 19 cases. Various morphological changes were seen in cases with PUO on bone marrow aspiration. Most common diagnosis was neoplastic changes, seen in 30% of cases, 14% cases show megaloblastic changes, 3.6% cases of iron deficiency was seen, 21% cases of reactive myeloid hyperplasia seen, haemophagocytosis seen in 5% cases, 6% cases show hypo cellular marrow. Among infections tuberculosis, malaria, Leishmaniasis and seen in 2%, 5% and 4%respectively. Normal marrow findings seen in 9%. Out of total 30 cases of neoplastic changes in bone marrow majority of them were acute myeloid leukemia seen in 36.67% cases. Acute lymphoid leukemia was the second common diagnosis constituting about 30% cases, all of them were seen in pediatric age group. Chronic myeloid leukemia and multiple myeloma were seen in 16.67% and 3.33% cases respectively. Myeloid dysplastic syndrome was seen in 13.33% cases. Conclusion: Bone marrow examination is an important investigation for etiological diagnosis of PUO. Overall most common causes of pyrexia of unknown origin were Neoplastic pathology. However most frequent cause in children were acute lymphoblastic leukemia and haemophagocytosis. Whereas in adults, the main causes were malignancies, reactive myeloid hyperplasia and megaloblastic anaemia. This study reflect light on the current spectrum of diseases causing pyrexia of unknown origin in this region.