In order to predict the steroid response in lipoid nephrosis (LN), we studied age, sex, proteinuria level, histological features and proteinuria selectivity index (SI; ratio between IgG and transferrin clearances) in 52 LN cases (minimal-change disease: n = 39; focal glomerulosclerosis+IgM nephropathy: n = 13). The multivariate analysis showed that age, sex and proteinuria level were not contributive, whereas histology and SI were. The predictive value of SI was much higher than that of histological type (McFadden's r2: 47% vs. 22%, p < 0.001). Thus, SI should be systemically assessed in idiopathic nephrotic syndrome for reviewing the pathologic classification obtained by histology. However, if its prognostic value is lower than that of selectivity, initial renal biopsy remains necessary for diagnosis in adults.