Corticosteroids enhance the production of the antiinflammatory cytokine IL-10 by T cells in vitro, but notably this response is impaired in cultures from SR asthmatic patients, implying an association between impaired IL-10 response and poor asthma control.2 This defect in steroid-induced IL-10 can be restored by the addition of the active form of vitamin D3 (1,25-hydroxyvitamin D3) into the culture, thus suggesting that vitamin D may play a role in controlling steroid responsiveness.3 These data are complemented by a number of independent clinical studies that highlight a high prevalence of vitamin D deficiency and insufficiency worldwide and its association with an increased incidence, severity, and poor control of asthma.4,5 The importance and status of a well-defined subset of Treg cells, as defined by the expression of the transcription factor forkhead box P3 (Foxp3), in SS and SR asthma are less well understood.1,6 Although 1,25-hydroxyvitamin D3 has been shown to enhance the frequency of human Foxp3+ Treg cells in vitro, no in vivo correlates of these data exist.7 The aim of the present study was therefore to investigate whether differences exist in the frequency of Foxp3+ Treg cells in the peripheral blood of SS and SR adult asthma patients, and the relationship between vitamin D status and the Foxp3+ Treg-cell compartment.