In INS, the histological appearance constitutes a classical prognostic element: minimal-change nephropathy (MCN) responds better to treatment than focal glomerulosclerosis (FGS) or IgM nephropathy (IgMN). However, this criterion is not consistent. We evaluated the prognostic value of the proteinuria selectivity index (SI): the ratio of IgG clearance to transferrin (Tf) clearance. Proteinuria was selective for an SI less than or equal to 0.01. In the 39 MCN, the SI ranged from 0.01 to 0.39 (median 0.10) and proteinuria was selective in 21 cases. In the 13 FGS and IgMN, the SI varied from 0.05 to 0.40 (median 0.22) and proteinuria was selective in 1 case (p less than 0.01 between these two groups). The SI ranged from 0.01 to 0.17 (median 0.07) for the 25 corticosensitive (CS) forms and from 0.08 to 0.40 (median 0.20) for the 27 corticoresistant (CR) ones (p less than 0.001). Twenty-four of the 30 MCN patients and 19 of 22 cases of selective proteinuria were CS. Multivariant analysis enabled the identification of variables predictive of the response to steroids. Age, sex and level of proteinuria had no such value. The predictive value of the SI was greater than that of the histological appearance (McFadden's R-square, 47 versus 22%, p less than 0.001). When the histological aspect was known, the SI provided additional precision, but the reverse situation was not true. The predictive curve of CS as a function of the SI was sigmoidal, therefore reflecting a homogeneous distribution, despite their different histological types.(ABSTRACT TRUNCATED AT 250 WORDS)