Chromosome banding studies carried out on bone marrow cells from a 16 year-old boy with an M1 acute nonlymphocytic leukemia (ANLL) revealed an unbalanced translocation involving chromosomes 1 and 10: der(10) t(1;10) (q21;q26) that results in a partial trisomy 1q between bands 1q21-1qter. Marker del(6)(q21) and trisomies of chromosomes 18, 21 and 22 were also observed. To our knowledge, this der(10) is the first to be reported in a patient with ANLL.