Abstract The results of cytogenetic analysis of bone marrow cells from a 13-year-old boy with acute lymphoblastic leukemia (ALL) are described. Initial analysis of aspirated bone marrow disclosed ALL FAB-L1 morphology, common (Ia+, cALLa+) immunophenotype and a complex abnormal karyotype. The majority of leukemic cells showed unbalanced translocations that resulted in complete trisomy of the long arm of chromosome #1 as the common denominator. The heterochromatic region of chromosome #1 (1qh) was associated with the telomeres of whole chromosomes #2, #13, and #16 in 28% of the 36 metaphases completely analyzed. Telomeric association is a very rare event and this is only the second known report of its occurrence associated with ALL wherein the 1qh region is attached to the telomeres of different chromosomes, resulting in trisomy of the whole 1q arm.