HLA antigens were examined in 146 children with idiopathic nephrotic syndrome (INS). These comprised 107 steroid-responsive cases, histologically characterised by minimal change glomerular lesions (MC), and 39 steroid-resistant patients with focal-segmental glomerulosclerosis (FSGS). In the MC groups, B8 was significantly increased as compared to controls (30% vs 18%, p less than 0.01), and this applied in particular to children with atopic features (38% B8 positive). In contrast to other reports, the frequency of B12 in steroid-responsive INS was not different from that of the control group. In FSGS, however, B12 was remarkably increased, especially in patients with a persistent or progressive nephrotic syndrome (45% vs 22%, p less than 0.025). These findings indicate that immunogenetic factors play a major role in INS, and that MC and FSGS are two different disease entities.