<p>Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, and previous research has shown that T cell-mediated immunity plays a key role in PD pathology. Programmed cell death protein 1 (PD-1) is a type I transmembrane protein that inhibits T lymphocyte inflammatory activity, and PD-1 deletion can reduce dopamine levels in mouse brains. However, the clinical status of PD-1 in PD patients remains obscure. To understand the clinical role of PD-1, flow cytometry (FCM) was used to assess peripheral blood mononuclear cells (PBMCs) from 16 patients with Parkinson's disease and 16 controls in our study. With respect to PD-1 expression on peripheral T lymphocytes, we found a statistically greater proportion of PD-1 on CD8⁺ T, CD4⁺ T, and T helper (Th) 1 cells in the PD group compared to the control group (<i>p</i> &lt; 0.05), and the proportion of CD4⁺ PD-1⁺ T cells was positively correlated with Parkinson's Disease Sleep Scale (PDSS) scores (r = 0.5277, <i>p</i> = 0.0454). Moreover, the PD patients had a lower percentage of CD3⁺ T cells (<i>p</i> = 0.0007) among PBMCs. Furthermore, the expression level of PD-1 was positively correlated with the expression of interferon-gamma (IFN-γ) in CD4⁺ T cell subsets from patients with PD (r = 0.6765, <i>p</i> = 0.0051) and total subjects (r = 0.4674, <i>p</i> = 0.0070), while there was no relationship between PD-1 and regulatory T (Treg) cells in the PD group. In conclusion, our results indicated that the PD-1 expression on peripheral T lymphocytes may be involved in the pathogenesis of PD.</p>