[Isolated trisomy 21 in blasts of an acute lymphoblastic leukemia].

A two years and four months old boy without evidence of Down's syndrome exhibited a trisomy 21 in his acute lymphoblastic leucemic cells. During remission this chromosomal change disappeared. The cytogenetic examination of malignant cells serves an important tool in diagnosis and follow-up of acute leucemia as well as bone marrow transplantation.