Fifty patients with dengue fever from Sept. 1987 to Jan. 1988 were studied for hematological features. The lowest blood counts values throughout the course of illness were Hb: 13.2 +/- 1.9 gm/dl, WBC: (2.77 +/- 1.63) x 10(3)/mm3, Platelet: (8.7 +/- 5.5) x 10(4)/mm3. Leukopenia (WBC less than 4000/mm3) was present in 38 (76%) of the cases and thrombocytopenia (platelet less than 10 x 10(4)/mm3) in 27 (54%) of the cases. Leukocytes reached nadir (1000-2000/mm3) at the 5th-6th day after fever onset, thrombocytes reached nadir (2.0-5.0 x 10(4)/mm3) at the 5th-7th day after fever onset. Bone marrow studies showed mild hypocellularity in the acute stage (less than 1 week) and normal cellularity in the convalescent stage (greater than 1 week). Megakaryocytes increased with various stages of maturation of megakaryocytes appearing in a majority of patients. Nuclear vacuolization of megakaryocytes could also be found. Bone marrow CFU-GM when performed within one week of illness showed no growth or low colony count, and was nearly normal after one week of fever onset. This study may suggest that leukopenia in dengue fever may be caused by virus-induced destruction or inhibition of myeloid progenitor cells. Thrombocytopenia may result from by destruction of peripheral platelet or bone marrow megakaryocytes by viruses which consequently reduce the platelet production.