A b s t r a c t We describe 15 patients (9 children) with precursor B-cell (pB) acute lymphoblastic leukemia (ALL) with surface immunoglobulin (sIg) light chain restriction revealed by flow cytometric immunophenotyping (FCI). The same sIg+ immunophenotype was present at diagnosis and in 3 relapses in 1 patient. In 15 patients, blasts were CD19+CD10+ (bright coexpression) in 14, CD34+ in 12, surface κ+ in 12, surface λ+ in 3; in 8 of 8, terminal deoxyribonucleotidyl transferase (The sIg light

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... in restriction in pB ALL might be present in neoplasms arising from the early, intermediate, and late stages of precursor B-cell maturation; sIg light chain restriction revealed by FCI does not necessarily indicate a mature B-cell phenotype, further emphasizing the importance of a multidisciplinary approach to diagnosing B-lymphoid neoplasms.