Background CD80 (also known as B7-1) is a co-stimulatory molecule that is expressed in biopsies and also excreted in urine in patients with minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS). CD80 is inhibited by the cytotoxic T-lymphocyte-associated-antigen 4 (CTLA4), which is mainly expressed on regulatory T cells (Tregs). Ineffective circulating Treg response is involved in the pathogenesis of nephrotic syndrome. In this study, we evaluated CD80 expression and infiltrating Tregs in children with MCD and FSGS. Methods Evaluation of CD80 expression and semi-quantitative evaluation of Tregs (FOXP3-positive CD4 T cells) were carried out in 31 kidney biopsies (12 MCD, 19 FSGS) with immunofluorescence and immunohistochemistry staining. Results All MCD sections were stained negative; whereas six out of 19 FSGS sections (all from steroid-resistant (SR) patients), including one from a Wilms' tumor 1 (WT1) mutation-positive FSGS patient, stained positive for anti-CD80 goat antibody, and negative for anti-CD80 rabbit antibody. FSGS biopsy specimens had significantly higher FOXP3-positive cells/mm(2) compared with MCD and control samples (P < 0.001). Biopsy samples from SR-FSGS patients (n = 12) with positive CD80 staining (n = 6) had significantly less Tregs (FOXP3-positive CD4 T cells) compared with CD80 (-) biopsies (n = 6; P = 0.004). Conclusion CD80 expression was not detected in the majority of the archival biopsy sections and the results were not consistent across the different antibodies. In the SR-FSGS sections, however, CD80-positive biopsies had decreased FOXP3-positive CD4 T cells, suggesting that a decreased anti-inflammatory milieu may be the cause of increased CD80 expression.