The purpose of this study was to compare the detection of trisomy 8 in myelodysplastic syndrome (MDS) patients with interphase fluorescence in situ hybridization (FISH) and cytogenetic karyotype analysis. Using Spectrum Green labeled chromosome 8 centromere probe, interphase FISH was established. The trisomy 8 clones were simultaneously detected in 48 MDS cases with FISH and conventional cytogenetic analysis (CCA). Results showed that the CCA revealed no significant difference of constitutional proportion between MDS-RA and MDS-RAEB with karyotypes of whole +8, partial +8 and one +8. With FISH, detectable rates were 66.1% for whole +8. Partial +8 and sole +8 were significantly higher than one +8 and complex +8, respectively. The percentages of trisomy 8 were similar in MDS-RA and MDS-RAEB. Trisomy 8 was detected in 1 of 15 specimens with normal or abnormal karyotype without trisomy 8 by FISH. There was linear correlation between the percentages of partial +8 detected by FISH and CCA. Two patients received CCA and FISH examination at diagnosis and during treatment, the percentage of trisomy 8 was increased with progress of disease. In conclusion, our results showed that FISH is a sensitive and accurate technique to detect trisomy 8 in MDS patients. It can provide contribute to diagnosis, assessment of curative effect and predicting progress of disease in MDS. Clone size of trisomy 8 does not related to classification of MDS, but sole +8 is seems to see in MDS-RA frequently.