Abstract: Objective: To determine the T lymphocyte subsets and natural killer(NK) cells in Guillain-Barré syndrome(GBS),and to explore the pathogenic mechanism of GBS.Methods: Blood samples from 12 patients with GBS at the acute stage of their disease and from 12 healthy subjects were studied by flow cytometry to define the circulating T lymphocyte,CD4⁺T cells,CD8⁺T cells,activated T cells,regulatory T cells(Treg),quantity of Th1 and Th2 and their relative proportion.Results: The proportions of total activated T cells,its subset activated CD4⁺ T cells and Th1 cells were increased(P<0.01) compared with the healthy control,while the proportion of Treg was decreased(P<0.01).Four of those 12 patients were detected again(15 days later) and the proportion of Treg at the late-acute stage of the disease was decreased(P<0.01) compared to the early-acute stage of the disease.The proportion of Th1 cells during the late-acute stage of the disease was decreased(P<0.05) compared to the early acute stage of the disease.Conclusions: The immune system in patients with GBS may be over-activated during the early acute stage of the disease.Immune injures in GBS may be induced by Th1 cells.Treg-cell-induced immune suppression is attenuated,and its correlation with the pathogenic mechanism of GBS and the recovery of nerve function requires further study.