Low anion gap occurs in a variety of disorders including hypoproteinemia. We evaluated anion gap in 28 children - 18 boys and 10 girls with minimal change nephrotic syndrome during 55 separate episodes of exacerbations. To assess the contribution of serum total proteins, albumin, cholesterol and immunoglobulin levels to low anion gap, a coefficient of correlation was calculated. Anion gap was significantly low (p less than 0.01) during exacerbations in nephrotic. The low anion gap correlated with low serum total proteins (less than 4 g/dl) and serum albumin levels (less than 1.5 g/dl) but not to serum cholesterol or immunoglobulin levels. Low total serum protein at patients blood pH, contributed fewer negative charges to anion gap thereby lowering the anion gap. The major cause of low anion gap in nephrotic syndrome is hypoproteinemia especially hypoalbuminemia.