We describe here the first case of acute lymphoblastic leukemia (ALL) with an isodicentric Philadelphia [idic(Ph)] chromosome. A 35-year-old man was diagnosed as ALL because of the infiltration of CD10(+)CD19(+)CD33(+)CD34(+) lymphoblasts in the bone marrow and the expression of p190-type BCR/ABL fusion transcript. Chromosome analysis showed 45,XY,der(7;12)(q10;q10),der(9)t(9;22)(q34;q11),idic der(22)t(9;22)(q34;q11). The idic(Ph) chromosome was spindle-shaped and supposed to be formed by two Ph chromosomes joined at their q terminals, whereas idic(Ph) chromosomes in chronic myelogenous leukemia (CML) have been shown to be fused at the satellite regions of p arms. The results indicate that the structure of idic(Ph) chromosomes appears to be different between ALL and CML. The patient did not respond to any chemotherapy and could not achieve remission. This chromosome aberration in ALL may suggest poor prognosis as observed in some cases of CML. Furthermore, considering other three reported cases, der(7;12)(q10;q10) may be one of the recurrent translocations in ALL.