Introduction. Leukemia is a heterogeneous group of hematological disorder which comprise several diverse and biologically distinct subgroups. Leukemia represents the 11th and 10th most frequent cause of cancer morbidity and mortality worldwide, respectively. Adequate data regarding the prevalence of leukemia are lacking in Ethiopia, particularly in the study area. This study is aimed to determine the prevalence of leukemia and associated factors among patients who have abnormal hematological parameters in Jimma Medical Center. Methodology. A facility-based cross-sectional study was conducted involving 332 patients who have abnormal hematological parameters. Complete blood count from venous blood was made with Sysmex autohematology analyzer (Sysmex XS-500i and XT-1800; Kobe, Japan). Peripheral blood morphology and bone marrow aspirate examination were done for each patient. Descriptive statistics for the prevalence of leukemia and multinomial logistic regression analysis to assess associated factors were executed with IBM SPSS version 25. Results. The prevalence of leukemia was 9.3%, while acute myeloid leukemia, Acute Lymphoid Leukemia, Chronic Myeloid Leukemia, Chronic Lymphoid Leukemia, Myelodysplastic Syndrome, and undifferentiated leukemia comprises 3.6%, 2.7%, 1.8%, 0.6%, and 0.3%, respectively. Older Age (
p
=
0.019
), being male (
p
=
0.047
), being anemic (
p
=
0.03
), and rural residency of a patient (
p
=
0.044
) were significantly associated with having acute myeloid leukemia. Conclusion. The prevalence of leukemia among patients who have abnormal hematological parameters in Jimma Medical Center is significant which needs further comprehensive investigations of the associated factors and predictors with more up to date diagnostic methods.