Erythrocyte sedimentation rate (ESR) was determined in 180 DHF patients and 70 patients with various viral and bacterial infections using the Winthrobe method. Seventy-seven percent of DHF patients had normal ESR (less than 20 mm/hour) and 15% had slightly elevated ESR value (between 21-30 mm/hour). The other 8% had ESR in the range of 31-49 mm/hour. Among 7 patients in the latter group, one had definite evidence of complicating urinary tract infection while the other 6 cases had anemia and severe bleeding. The mean ESR in DHF patients (10.71 mm/hour) was significantly lower than the mean values in other groups of patients with viral infection (20.46 mm/hour), bacterial infection (34.81 mm/hour) and miscellaneous illnesses (35.29 mm/hour). The mean ESR in shock cases was 7.63 mm/hour while in non-shock cases it was 13.87 mm/hour and they are statistically different (p less than 0.05). The mean ESR during the time of shock was lower than in the pre-shock and post-shock period. Hemoconcentration, low level of albumin and fibrinogen and the presence of disseminated intravascular clotting (DIC) in a majority of DHF patients are most likely responsible to this observed lower ESR especially during shock period. The determination of ESR, which is simple, is therefore useful in differentiating DHF from bacterial infections and dengue shock syndrome from septic shock.