OBJECTIVE To investigate the change of lymphocyte subsets before and after chemotherapy in colorectal carcinoma patients. METHODS Twenty-one peripheral blood lymphocyte subsets from 62 colorectal carcinoma patients before and after FOLFOX4(including oxaliplatin, 5-fluorouracil and leucovorin) , FOLFRI(including irinotecan, 5-fluorouracil and leucovorin) , or XELOX(including oxaliplatin and capecitabine) regimen chemotherapy were examined by flow cytometry.The differences of these lymphocyte subsets were analyzed. RESULTS After chemotherapy, the percentages of CD3(+), CD3(+)CD8(+), CD29(+), CD4(+)CD29(+), and CD4(+)CD25(+) cells in peripheral blood of colorectal carcinoma patients increased significantly, while the percentages of CD19(+) and human leukocyte antigen(locus) DR(HLA-DR) (+) cells decreased significantly(P<0.05) .The results of subgroup analysis showed that the patients' CD3(+)CD8(+) and CD4(+)CD25(+) cells increased significantly, CD19(+) and HLA-DR(+) cells decreased significantly after FOLFOX4 regimen chemotherapy(P<0.05) ;CD3(+)CD8(+) cells increased significantly and CD19(+) cells decreased significantly after XELOX regimen chemotherapy(P<0.05) ;while after FOLFRI regimen chemotherapy, there were no significant changes in all 21 lymphocyte subsets(P>0.05) . CD3(+), CD3(+)CD8(+), memory T lymphoctye(45RO(+)) , and CD4(+)CD45RO(+) cells increased significantly(P<0.05) in patients who received no more than 4 cycles of chemotherapy. However, in patients that received 5 to 8 cycles and more than 9 cycles chemotherapy, we only found significant decrease of HLADR(+) cells and significant increase of CD29(+) cells, respectively(P<0.05) . CONCLUSIONS The humoral immunity is attenuated after chemotherapy in colorectal carcinoma patients. FOLFOX4 may suppress the cellular immunity.Chemotherapy that is less than 4 cycles will strengthens the cellular immunity by modulating body immunity arrangement;however, along with the increase of chemotherapy cycles, the cellular immunity gradually declines in these patients.