We determined the number and functional status of CD4⁺CD25high regulatory T cells (Treg) in blood samples from patients with metastatic carcinoma, and evaluated their sensitivity to a single intravenous infusion of cyclophosphamide. Treg numbers were significantly higher in 49 patients with metastatic cancer (9·2% of CD4⁺ T cells) compared to 24 healthy donors (7·1%). These cells expressed the transcription factor forkhead box P3 (FoxP3), glucocorticoid-induced tumour necrosis factor receptor family-related protein (GITR) and intracellular CD152, and demonstrated a suppressive activity in vitro against CD4⁺CD25⁻ autologous proliferation. At a single intravenous infusion, cyclophosphamide failed, in association with a non-specific immunotherapy by intratumoral bacille Calmette-Guérin (BCG), to modulate significantly Treg numbers or function. Metastatic cancer is associated with an expansion of peripheral blood CD4⁺CD25highFoxP3⁺GITR⁺CD152⁺Treg cells whose immunosuppressive properties do not differ from those of healthy subjects. Moreover, cyclophosphamide administration may not represent an optimal therapy to eliminate Treg, which further underlines the need to identify specific agents that would selectively deplete these cells.