Urine concentrations of two high molecular weight proteins, albumin and IgG, and two low molecular weight proteins, alpha1-microglobulin (A1M) and retinol-binding protein (RBP), were measured in 657 healthy children from birth to 18 years of age. The urinary levels of RBP and A1M suggest an age-dependent decline, whereas the levels of albumin and IgG show an uneven distribution. Reference values for albumin, IgG, A1M, and RBP for each group are reported.