A retrospective observational study of dengue fever was performed, including 210 patients (male:female ratio 1.6:1, ages 6-74 y, mean 29.7 y) attending the Aga Khan University Hospital, Karachi from January 2001 to December 2006. All included patients proved dengue IgM antibody positive. Of these, 19 (9%) showed increased haemoglobin/haematocrit levels on admission which remained elevated in 4 (2.1%) at the time of discharge. 56 patients (26.6%) had leucopenia and neutropenia and 77.1% (161) had thrombocytopenia at the time of admission; 2.5% (5) and 16.7% (35) had deranged PT and APTT, respectively. Atypical lymphocytes were seen in 109 patients (52%). Platelet transfusion was given in 45 (22.1%) cases. The majority of patients were discharged without any adverse sequelae. The fatality rate was 3.3% (n = 7) and these patients died of dengue shock syndrome, while 196 (93.3%) recovered completely. Haematological parameters are an important clue and should be tested when a patient presents with symptoms suggestive of dengue fever.