Background: Precursor B-cell acute lymphocytic leukemia (ALL) with surface immunoglobulin light chain expression is a rare disease entity. The differential diagnosis is difficult but critical for disease management. Aims: We report 2 cases (1 adult and 1 infant) of precursor B-cell ALL who presented at diagnosis with surface immunoglobulin light chain expression revealed by flow cytometric immunophenotyping and discuss its clinical significance. Case Report: The 2 patients presented with nonspecific symptoms such as fever, pallor, fatigue or lymphadenopathy. Flow cytometric immunophenotyping showed that both patients expressed CD34/CD19/CD10/CD22/CD9/HLA-DR/CD38/CD123/CD13 (partial) and had unexpected single λ light chain expression. Cytogenetic analysis revealed t(9;22)(q34;q11) in the adult patient and normal karyotype in the infant. Both cases were diagnosed and managed as precursor B-ALL, and the patients showed good response to treatment regimens. Conclusion: We describe 2 cases of precursor B-ALL with unexpected surface light chain expression. The exceedingly rare immunophenotypes have diagnostic implication for immunophenotyping of this malignancy. Treatment regimens for precursor B-cell ALL are suitable for such cases.