Objective:To investigate the expression change of human leukocyte antigen (HLA) class Ⅰ on human peripheral blood mononuclear cells (PBMCs) at both mRNA and protein levels,and to evaluate its roles in the development of colorectal cancer (CRC).Methods:In the present study,50 patients with CRC,35 patients with benign colorectal lesion and 42 healthy volunteers were enrolled.Expression levels of HLA class Ⅰ mRNA and protein were determined using real-time quantitative reverse transcription PCR (RT-PCR) and flow cytometry analysis,respectively.Results:The expression levels of HLA class Ⅰ mRNA and proteins were not influenced by age and gender.The relative ratios of HLA class Ⅰ mRNA were 0.99±0.27 in healthy controls,0.76±0.19 in benign patients,and 0.48±0.21 in CRC patients.Mean fluorescence intensities of HLA class Ⅰ were 145.58±38.14 in healthy controls,102.05±35.98 in benign patients and 87.44±34.01 in CRC patients.HLA class Ⅰ on PBMCs was significantly down-regulated at both mRNA and protein levels in patients with stage Ⅲ and Ⅳ CRC.CRC patients with lymph node metastasis also showed a decreased HLA class Ⅰ expression at protein level.Conclusion:HLA class Ⅰ expressions on PBMCs are associated with staging of CRC and lymph node metastasis.Monitoring the expression of HLA class Ⅰ on PBMCs may provide useful information for diagnosis and metastasis judgement of CRC.