We report an interesting case of chronic myelomonocytic leukemia (CMML) with pentasomy 8q resulting from the duplication of isochromosome 8q in a 47-year-old male. His blood picture and myelogram showed CMML and the chromosome study, using R-banding and G-banding techniques, revealed a karyotype of 47,XY,-8,+i(8)(q10)x2. Dual-color fluorescence in situ hybridization (FISH) studies with a #8 centromeric probe and a locus-specific probe for C-myc gene completely confirmed the result of the conventional cytogenetic method. Reverse transcription polymerase reaction (RT-PCR) revealed no BCR/ABL fusion transcript. Hydroxyurea and 6-mercaptopurine therapy did not induce a complete remission and five months later he died of exacerbation of his disease. On reviewing another two cases with pentasomy 8q in the literature, we feel that pentasomy 8q, when present as a sole anomaly, may play a specific role in leukemogenesis and in determining the clinical characteristics such as monocytic involvement and poor prognosis.