The complex chromosome abnormalities (CCAs) were one of the most important poor prognostic risk factors in acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). Chromosome analysis using classical cytogenetic banding techniques fails to completely resolve complex karyotypes and cryptic translocations. The technique of multiplex fluorescence in situ hybridization (M-FISH) allow for the simultaneous visualization of all chromosomes of a metaphase in a single hybridization step and thereby enable to comprehensively analyze complex karyotypes and the identification of new and cryptic translocations. To investigate the value of M-FISH in the detection of complex karyotypic abnormalities of AML and MDS. M-FISH was used in combination with interphase-FISH to study 24 cases of AML and MDS with CCAs showed by R-banding of conventional cytogenetics (CC). In 14 cases of AML with CCAs, 4 gains of whole chromosome and 4 losses of whole chromosome were confirmed by M-FISH, while 12 losses of whole chromosome were revised as derivative chromosomes resulted from various structural aberrations. 26 derivative chromosomes and 19 marker chromosomes were characterized precisely by M-FISH. Most of them were unbalanced translocations, including 2 complex t(8;21), which have not been previously described:t(8;21), der(8) t(8;21) (8pter→8q22::21q22→21qter), der(21) t(8;21;8) (8qter→ 8q22::21p13→ 21q22::8q22→ 8qter) and t(21;8;18;1), der(8) t(8;21) (8pter→ 8q22::21q22→ 21qter), der(21) t(21;8;18;1) (21p13→ 21q22::8q22→ 8q24::18?::1q?q?). In 10 cases of MDS, 37 kinds of structural rearrangements were detected by M-FISH including insertion, deletion, translocation and derivative chromosomes, and among them 34 kinds were unbalanced rearrangements, only 3 were balanced rearrangements including t(6;22)(q21;q12), t(9;19)(q13;p13) and t(3;5)( ?;?), 7 abnormalities were never reported before. The CCAs invloved nearly all chromosomes, of which the chromosome 17, 5 and 7 were invloved more frequent than the rest. Chromosomes 5, 17, 7 were involved in 15 cases (62.5%), 12 cases (50%) and 6 cases (25%) respecrively. We conclude that M-FISH could refine CCAs of AML and MDS patients, find or correct the missed or misidentified aberrations by CC analysis. Our findings confirm that M-FISH is a powerful tool to characterize complex karyotypes in AML and MDS.