We investigated the utility of a standard urinary protein electrophoresis (UPEP) to distinguish among three common variants of childhood idiopathic nephrotic syndrome (NS). The UPEP was performed on 66 urine samples obtained during a disease relapse in 43 children and adolescents with idiopathic NS. There were 15 children with minimal change disease (MCD), 11 with IgM nephropathy (IGMN) and 17 with focal segmental glomerulosclerosis (FSGS). Fourteen of the 26 children (54%) with MCD or IGMN and 16/17 (94%) of the patients with FSGS manifested a frequently relapsing or steroid dependent course. The mean percent albumin and gamma globulin excretion in the UPEP in patients with MCD and IGMN were 75.5 and 2.9 versus 72.6 and 3.9, respectively (p = NS). Both patterns were significantly different from that observed in FSGS, albumin 62.2%, gamma globulin 7.1% (p less than 0.005). Although the percent gamma globulin excretion was inversely related to GFR in children with FSGS, this measurement exceeded a 4.3% cutoff in 9 of these patients while their GFR was normal (less than or equal to 80 ml/min/1.73M2). Therefore, we recommend the use of the UPEP as a marker of urinary protein selectivity and to monitor children with high-risk nephrotic syndrome i.e., those with a frequently relapsing or steroid dependent clinical course, for histological transitions.