Summary Thyroid-associated ophthalmopathy (TAO) is a common yet poorly understood component of Graves' disease involving inflammation, congestion and soft tissue remodelling of the orbit. Unlike most autoimmune disorders, TAO has variable severity but follows a predictable course and is usually self-limited. The objective of this study was to investigate the phenotypic profile of peripheral blood mononuclear cells in euthyroid patients with TAO. The study was a prospective, consecutive analysis of the peripheral blood mononuclear cell phenotype in patients with TAO and normal controls. We demonstrate that the fraction of T cells expressing CD69, CD25 or CXCR4 is significantly greater in patients with TAO compared to control donors. In addition, the fraction of CD19+ CD25+ B cells is significantly greater. We did not find differences between the two groups of subjects in monocytes expressing these markers. There is a phenotypic shift in peripheral blood lymphocytes associated with TAO that appears durable and persists beyond the hyperthyroid phase of Graves' disease. These changes may support the immune reaction provoking orbital disease development.