Objective:To investigate the level and clinical significance of peripheral blood CD4 +T cell subpopulations in late-onset systemic lupus erythematosus (SLE) patients. Methods:This study included 260 SLE patients hospitalized in the Rheumatology and Immunology Department of the Second Hospital of Shanxi Medical University from January 2016 to December 2021: of whom 58 and 202 were late- (≥50 years) and adult-(18~49 years) onset patients. This study also included 160 subjeces as healthy controls(HCs), of whom 35 and 125 were Control Group 1 (≥50 years) and Control Group 2 (18~49 years). Peripheral blood CD4 +T lymphocyte subsets of these participants were assessed by flow cytometry. The clinical data of all patients and healthy controls (HCs)were recorded. The differences between the groups were analyzed by Mann-Whitney U test or χ2 test. Results:(1)The time of diagnosis of late-onset SLE was longer than that of adult-onset SLE [Median time: 5.0 (2.0, 24.0)months vs 3.0 (1.0, 7.3)months, Z=-3.13, P=0.002]. Compared with adult-onset SLE, the SLEDAI score of late-onset SLE was lower [12.0 (8.0, 15.2) vs 14.0 (10.0, 18.0), Z=-2.12, P=0.034]. Some manifestations occurred more frequently in late-onset SLE, such as weight loss, nausea, abdominal pain, cerebral infarction, interstitial pneumonitis, Sj?gren′s syndrome and infection. The manifestations of skin and mucos a occurred less frequently in late-onset SLE. (2)CD4 +T cell subpopulations: ①The absolute counts of Treg, Th17, Th1 and Th2 cells in the peripheral blood of patients with late-onset SLE were significantly lower than those of HCs [Treg: 10.94 (6.14, 19.23) vs 32.65 (28.07, 41.65), Z=-6.79, P<0.001; Th17: 3.43 (0.94, 5.64) vs 6.13 (3.77, 7.82), Z=-3.24, P=0.001; Th1: 36.02 (10.80, 76.38) vs 128.70(89.82, 159.89), Z=-5.29, P<0.001; Th2:3.56 (1.56, 6.06) vs 8.25 (4.69, 12.98), Z=-4.57, P<0.001]. The ratio of Th17/Treg cells was higher than that of HCs[0.28(0.13, 0.59) vs 0.17 (0.12, 0.28), Z=-2.38, P=0.017].②The absolute counts of Treg, Th17, Th1 and Th2 cells in peripheral blood of patients with adult-onset SLE were significantly lower than those of HCs [Treg: 10.28 (5.37, 17.04) vs.30.19 (21.20, 39.75), Z=-11.28, P<0.001; Th17: 3.44 (1.84, 6.14) vs 6.48 (4.23, 10.66), Z=-6.53, P<0.001; Th1: 29.59(15.14, 56.81) vs 90.75(42.67, 162.00), Z=-7.01, P<0.001; Th2: 2.74 (1.62, 4.77) vs 8.25 (4.75, 11.99), Z=-9.91, P<0.001]. The ratio of Th17/Treg was higher than that of HCs[0.35 (0.17, 0.65) vs 0.23(0.14, 0.37), Z=-3.89, P<0.001].③The ratios of Th17/Treg in patients with late-and adult-onset SLE were higher than those of HCs. The ratio of Th17/Treg was the highest in adult-onset SLE patients. Conclusion:Patients with late-onset SLE have reduced numbers of Treg cells and the immune imbalanced of Th17/Treg. However, the immune imbalance of Th17/Treg in late-onset SLE patients is milder than that in adult-onset SLE patients, which may be related to lower disease activity.