Background: Dengue in early infancy could progress rapidly in to the critical phase with subtle clinical manifestations without classical laboratory changes. Early recognition of plasma leakage is crucial to prevent morbidity and mortality . We report a 5-month-old infant who entered the critical phase of dengue at the beginning of day 2 of illness with preserved haemodynamics, without dropping platelet below 100,000/mm3 or rising trend of heamatocrit. Case presentation: 5-month-old infant, presented with fever for one day . There was no history of previous dengue infection, no exposure to steroids, non-steroidal anti-inflammatory drugs or paracetamol overdose. On admission, had a positive NS1 antigen test with a platelet count of 160,000/mm3. hematocrit was 35. On early day 2 of illness, the infant remained febrile and appeared unwell but continued to maintain stable vital signs. However, significant hepatic tenderness was noted on examination. The platelet count had dropped to 147 ,000/mm³ with a stable PCV of 35. The ultrasound scan was performed and demonstrated free fluid in the abdomen and a right-sided pleural effusion, confirming the critical phase of denguehaemorrhagic fever. Meticulous critical phase dengue monitoring was commenced. Platelet count dropped to 67 ,000/mm³ on day 3 and reached a lowest value of 19,000/mm³ on day 5. The baby recovered fully without bleeding manifestations, shock, or organ dysfunction. Conclusion: This case highlights the importance of meticulous monitoring and earlyultrasonography in infants with dengue, even when clinical and laboratory parameters do not suggest entering to critical phase in contrast to rest of the paediatric population.