Introduction: Dengue haemorrhagic fever (DHF) causes significant paediatric morbidity and mortality . The critical phase is characterized by plasma leakage which may be difficult to detect early as clinical and haematological changes can be subtle or delayed.Ultrasonography is widely used to detect plasma leakage early and to estimate time of entry into critical phase. Objectives: The primary objective was to assess the correlation between ultrasonographic findings and haematological parameters in determining the onset of critical phase. Additionally , demographic characteristics and frequency of ultrasonographic findings were described. Methods: A retrospective, observational study was conducted among children with DHF admitted to ward 02, Colombo North Teaching Hospital, Ragama, Sri Lanka, from 01/03/2017 to 31/08/2017 . Data was extracted from medical records and analyzed using R statistical software. Results: 57 children were included. 56.1% were male. The most frequent ultrasonographic finding was gallbladder wall oedema (93.0%), followed by ascites (78.9%). Sonographic evidence of leaking was significantly higher among patients with markedly reduced platelet counts. No statistically significant difference was observed in detection of plasma leakage when ultrasonography was compared with clinical monitoring charts (P=0.354), or haemoconcentration (p=0.424). Ultrasound detected the critical phase earlier than haemoconcentration in 63.3% Conclusions: Ultrasonographic evidence of plasma leakage is higher in patients with decreased platelet count consistent with previously published data. Ultrasonographic detection of onset of critical phase is comparable to the detection by haemoconcentration. Ultrasonography facilitates earlier recognition of plasma leakage enabling timely intervention and improved outcomes.