Introduction: Acute Basophilic Leukemia (ABL) is an unusual hematological disease and a rare subtype of acute myeloid leukemia (AML). First described in 1906 as “Basophilic Leukemia”, the pathophysiology of this neoplasm remains unclear, with a worldwide incidence of less than 1% among AML. The few cases reported are generally associated with poor prognosis. Clinical case: A 30-year-old male was admitted to our institution on suspicion of acute promyelocytic leukemia (APL). At presentation he had gingival bleeding, hematuria and asthenia in the last two weeks which were associated with bicytopenia on blood tests. However, there was no coagulation disorder and he had a complaint of disseminated pruritus. The blood count revealed a hemoglobin level of 7.3g/dL, 10 × 109/L platelets, 8.5 × 109/L leukocytes, with 3.24 × 109/L neutrophils, 0.11 × 109/L basophils and 43% blasts. In peripheral blood microscopy, the blasts were large aberrant cells with basophil-like dark coarse granules. Bone marrow aspiration showed 31% of blasts and an increased number of eosinophils and basophils. A double population of blasts were seen in flow cytometry (FC). The immunophenotyping (IP) of the larger one was CD117 weak, CD123+, CD33++, CD11b+, CD13++, and negative HLA-DR, CD56, CD7, CD15, CD64, CD14 and CD34. The second population of blasts expressed CD117++, CD34 weak, HLA-DR weak, CD33++, CD13 weak, CD64 partial weak and negative CD123, CD19, CD7, CD14, CD15 and CD11b. Conventional cytogenetics found a normal karyotype, 46XY. The molecular evaluation found no recurrent mutations (RUNX1-RUNX1T1, PML-RARA, CBFb-MYH11, BCR-ABL, FLT3-ITD and NPM1 were all negative). During evaluation for disease remission, the assessment of minimal residual disease by FC detected 1.8% of myeloid blast cells. At that time, we decided to expand phenotypic markers to basophils and the blasts expression profile was the same of the larger population of blasts at diagnosis added to the positivity for CD203+, CD9+ and CD22 weak, thus corroborating the IP of ABL. Treatment with protocol 7+3 (idarubicin 12 mg/m2 + cytarabine 100 mg/m2) was performed, followed by consolidation cycles of HDAC (3 g/m2) while he waits for an allogeneic stem cell transplant from his haploidentical sister. Discussion/Conclusion: According to World Health Organization classification, ABL is considered an independent entity that is included in the AML, NOS category. Therefore, a careful morphological analysis, immunophenotyping with basophilic myeloid markers and the absence of recurrent genetic abnormalities are necessary for an accurate diagnosis. Commonly, patients with ABL may present complications related to high levels of histamine due to the degranulation of these cells, such as skin rash, peptic ulcers and anaphylaxis. In this case, despite the morphology being suggestive of basophilic blasts, the first FC were not directed to detect specific basophilis lineage markers due to the rarity of this disease and the preliminary misdiagnosis of APL. However, upon reviewing the case, the FC panel profile was expanded with specific markers for ABL (CD9, CD203, CD22) being evidenced. Remarkably, our patient had pruritus, and from that we can retrospectively infer an association with hyperhistaminemia, but this test was not available. This case illustrates the importance of clinical practice based on anamnesis, morphology and biological markers to establish a rare diagnosis.