We describe the rare finding of a T-cell acute lymphoblastic leukemia (T-ALL) and a pretreatment bone marrow karyotype mosaic for four distinct cell lines in a 4-year-old boy. G-banding analysis of metaphase cells identified a hyperdiploid cell line (52 chromosomes) trisomic for chromosomes 6, 9, 11, 13, 19, and 22. Fluorescence in situ hybridization (FISH) analysis demonstrated that these hyperdiploid cells were missing all three copies of the CDKN2A locus (alias p16/Ink4) at 9p21. FISH analysis of interphase nuclei identified two abnormal cell lines: the majority of cells with homozygous deletions of the CDKN2A locus and some with a heterozygous deletion. In addition, a normal signal pattern was identified in a few cells. This case represents a rare case of hyperdiploidy in T-ALL, and characterizes the clonal evolution of the 9p21 deletion leading to the abnormal karyotype.