Acute promyelocytic leukemia (APL) is a specific type of acute myelogenous leukemia characterized by a typical morphology and by a translocation between chromosomes 15 and 17, t(15;17)(q22;q21). Because this translocation is not found in other subtypes of acute myelogenous leukemias, it has become an important marker in the diagnosis and treatment of APL. Here we report a case with apparent absence of t(15;17) cells by cytogenetic techniques at diagnosis. At relapse, a metaphase FISH analysis (not dependent on chromosomal quality), followed by G-banding, was performed. t(15;17) was detected in 60% of the metaphases. In 30% of metaphases, an additional t(6;17) was detected involving the other chromosome 17 at band q21. In parallel, a molecular study (using the Southern blotting technique) was carried out and a single molecular rearrangement of the RAR-alpha gene was detected implying that RAR-alpha was not rearranged in the t(6;17) translocation.