Introduction: Acute febrile illness (fever of short duration 2-3 week) is a common presentation. This study is intended to know thrombocytopenia in patients of acute fever, its severity, cause and recovery. Aims and Objective: To clinically study thrombocytopenia in Aute Febrile Illness and the aetiology Material and Methods: The study was conducted on 100 patients who presented with fever of short duration in Darbhanga Medical College & hospital Results: Thrombocytopenia is very common in fever of short duration in all age groups ranging from 10 years to 80 years. Approx. 40 % patients have platelets between 50,000 to 1,00000 mm3. In 45 % patients cause was mostly viral fever (not dengue). In 30% enteric fever, 10% dengue fever, 8 % malaria and 7 % septicemia.> 98 % patients had good recovery. Conclusions: Thrombocytopenia is quite common in patients presenting with fever of acute onset (2-3 week duration) and the main cause is unknown not malaria, enteric fever or dengue.