The clinical heterogeneity of idiopathic nephrotic syndrome (INS) in childhood may reflect different mechanisms of disease that are as yet unclear. Here, we evaluated the association between an atypical presence of IgM on the surface of T cells (T-cell IgM) and the response to steroid therapy in a total of 153 pediatric patients with INS in different phases of disease. At disease onset, T-cell IgM median levels were significantly elevated and predictive of risk of relapse in 47 patients. They were also significantly increased comparing 58 steroid-dependent (SD) to 8 infrequently relapsing and 14 frequently relapsing patients, especially during relapse, whereas they were within the normal range in 7 genetic steroid-resistant (SR) patients. T-cell IgM in vivo was not affected by the amount of total circulating IgM, nor by concomitant acute infections or oral immunosuppression. However, it was affected by rituximab (RTX) treatment in 21 SD patients. By in vitro experiments, elevated T-cell IgM was not influenced by total circulating IgM levels or by the presence of other circulating factors, and there was no distinctive antigen-specificity or atypical IgM polymerization. Rather, we found that increased T-cell IgM correlates with reduced IgM sialylation, which influences T-cell response to steroid inhibition and T-cell production of podocyte-damaging factors. Thus, the atypical presence of IgM on the surface of T cells may predispose a subset of steroid-sensitive (SS) pediatric patients with INS to a poor response to steroid therapy since disease onset.