Background Regulatory T cells (Tregs), together with tolerogenic dendritic cells (tDCs) are involved in maintaining peripheral tolerance. A recent report suggested both Tregs and tDCs may be pathogenic in granulomatous skin disorders. Aim To examined the expression of CD39 on granuloma-composing cells and Foxp3-positive Tregs in the skin in two representative granulomatous diseases, sarcoidosis and granuloma annulare (GA). Methods We immunohistologically examined expression of CD39 on granuloma-composing cells and expression of Foxp3 on CD4+ or CD25+ cells in fixed sections of lesional skin from 16 patients with sarcoidosis and five patients with GA. Results The granuloma-composing cells expressed CD39 in both sarcoidosis and GA. Significant numbers of CD4+ Foxp3+ Tregs were present diffusely throughout the granulomatous tissues in sarcoidosis, whereas Tregs in GA existed only at the peripheral lesion of palisading granulomatous tissue. Conclusions There was infiltration of increased numbers of Foxp3+ Tregs around the CD39+ granuloma-composing cells in both GA and sarcoidosis. Click here for the corresponding questions to this CME article.