SummaryThe aim of this study was to quantify and evaluate the forkhead box P3 (FoxP3) expression regulatory T cells in new-onset systemic lupus erythematosus (SLE) patients before and after treatment. Forty-four newly diagnosed and untreated SLE patients, including 24 with active disease (SLEDAI Ն 10) and 20 with inactive disease (SLEDAI < 5), were enrolled in this study. Twenty-one age-and sex-matched healthy volunteers were also included as controls. Peripheral blood samples were collected and mononuclear cells isolated. The expression of CD25 and FoxP3 in CD4 + T cells were analysed with flow cytometry. CD4+ CD25 + (3·95-13·04%) and CD4 + CD25 high (0·04-1·34%) T cells in peripheral blood in untreated patients with new-onset active lupus were significantly lower than that in the patients with inactive lupus (7·27-24·48%, P < 0·05 and 0·14-3·07% P < 0·01 respectively) and that in healthy controls (5·84-14·84%, P < 0·05). Interestingly, the decrease in CD4 + CD25 high T cells was restored significantly in patients with active lupus after corticosteroid treatment. There was, however, a significantly higher percentage of CD4 + FoxP3 + T cells in patients with active (5·30-23·00%) and inactive (7·46-17·38%) new-onset lupus patients compared with healthy control subjects (2·51-12·94%) (P < 0·01). Intriguingly, CD25 expression in CD4 + FoxP3 + T cells in patients with active lupus (25·24-62·47%) was significantly lower than that in those patients with inactive lupus (30·35-75·25%, P < 0·05) and healthy controls (54·83-86·38%, P < 0·01). Most strikingly, the levels of FoxP3 expression determined by mean fluorescence intensity in CD4 + CD25 high cells in patients with active SLE were significantly downregulated compared with healthy subjects (130 Ϯ 22 versus 162 Ϯ 21, P = 0·012