Abstract Dengue fever is the most common cause of hospitalization in otherwise healthy person. The incidence rate increases significantly every year, from 0.05 per 100.000 inhabitants in 1968 to 35-40 per 100.000 inhabitants in 2013. Thrombocytopenia and leucopenia are the most commonly reported changes, whether they are caused by bone marrow suppression or peripheral destruction. Anemia has also been observed to occur in dengue infection with unknown mechanism. Immature Reticulocyte Fraction (IRF) is a parameter reflecting the most immature reticulocyte fraction and it can identify the earliest stage of erythropoiesis disorder. This research aimed to determine the mechanism of erythropoiesis disorders that led to anemia using IRF parameter in the various clinical phases of dengue fever. This study was a comparative analytical research using secondary data derived from the Dengue-associated Endothelial Cell Dysfunction and Thrombocyte Activation (DECENT) research. The baseline characteristic data consists of sex, age, level of hemoglobin, hematocrit, leucocyte, and thrombocyte, also the IRF. The patients were grouped into the fever, critical, recovery, and convalescent phases, plus healthy control group. The data was analyzed using the Kolmogorov-Smirnov normality test, followed by Friedman test and Mann-Whitney post hoc test. There were 244 research subjects and the median age was 24 (14-67), with similar ratio of male and female. The median IRF for all the research subjects was 4.8% with an IQR of 2.4-8.1%. The fever-phase group showed a median of 1.8% with an IQR of 0.5-2.85%. The critical-phase group showed a median of 3.6% with an IQR of 1.8- 5.0%, while the median for the recovery-phase group was 7.05% with an IQR of 4.08-11.85%. The convalescent-phase group showed a median of 7.3 % with an IQR of 3.95-9.3%, and the healthy-control group showed a median of 4.1% with an IQR of 2.2-6.6%. There was a significant difference in IRF between the groups (p<0.05). Immature Reticulocyte Fraction in fever phase was significantly different with IRF in other phases and healthy controls (p<0.05). In the critical phase, IRF was not significantly different from the healthy controls (p=0.218). Summarizing, there are changes in erythropoiesis activity in various clinical phases of dengue infection. Erythropoiesis suppression occurred mainly during the fever phase and it started to restore in the critical phase. In the recovery and convalescent phases, the erythropoiesis activity was increased.