Aplastic anemia (AA) may sometimes precede the diagnosis of acute lymphoblastic leukemia (ALL) in children. Such presentation of ALL is externally rare in adults and until now only few such cases have been reported. We present a 40-year-old male with ALL common type, which developed 14 months after the diagnosis of severe AA, successfully treated with corticosteroids. ALL was treated with standard induction chemotherapy but remission has not been achieved. The patient died 6 weeks after the diagnosis of ALL because of central nervous system bleeding. The pattern of IgH gene rearrangement analyzed with PCR method in bone marrow from the period of AA diagnosis and in peripheral blood mononuclear cells from ALL diagnosis showed two different monoclonal IgH configurations as the results of biallelic monoclonal rearrangement of IgH genes. The observed bands in both specimens were identical and indicated that leukemic cells originated from B-cell progenitor were also present in the bone marrow when AA was diagnosed. We suggest that molecular analysis of monoclonality in patients with AA may be important for proper selection of the rare cases of ALL first presenting as marrow aplasia.