CD80 is expressed on all antigen-presenting cells and is present on podocytes in a number of experimental models of nephrotic syndrome. We tested whether urinary soluble CD80 increased with idiopathic minimal-change disease (MCD). We collected urine and serum samples from patients with MCD in relapse and in remission, patients with nephrotic syndrome resulting from other glomerular diseases (FSGS, membranoproliferative glomerulonephritis, IgA nephropathy, and membranous nephropathy), patients with systemic lupus erythematosus, and normal control subjects. Urinary concentrations of soluble CD80 in patients with relapsed MCD were significantly higher compared with those observed in patients with MCD in remission, other glomerular diseases, and systemic lupus erythematosus with and without proteinuria and healthy control subjects. Urinary concentrations of soluble CTLA-4, which is a negative regulator of CD80, were not statistically different in patients with relapsed MCD compared with those in remission. The urinary soluble CD80/CTLA-4 ratio was >100-fold higher in patients with relapsed MCD compared with those in remission (P < 0.008). In contrast, serum concentrations of soluble CD80 and CTLA-4 did not distinguish patients with MCD in relapse and in remission. In conclusion, urinary soluble CD80 is elevated in idiopathic MCD, which could be relevant to both diagnosis and pathogenesis.