Background: Aberrant phenotypes in acute leukemia (AL) have variable frequency and their prognostic relevance is controversial and dissimilar results have been reported by different groups. Objectives: To determine frequency of aberrant marker expression in acute leukemia at South Egypt Cancer Institute (SECI) and to relate these expression with patient outcome and cytogenetic abnormality. Patients and Methods: A retrospective study was conducted at SECI over a period of ten years. Total 1134 AL patients of both genders and all age groups were included in the study, of them 458 were diagnosed as acute myeloid leukemia (AML), 494 were diagnosed as B-acute lymphoblastic leukemia (B-ALL), 157 were diagnosed as T-ALL and 25 were diagnosed as biphenotypic AL (BAL). Results: Aberrant phenotypes were found in 91 (19.9%) cases of AML, 148 (30%) cases of