Objective. To investigate the association between immunogobulin G (IgG) in urine and serum, and disease severity in patients with idiopathic membranous nephropathy (IMN). Methods. A total of 280 patients with IMN were enrolled in this study. Serum and urine IgG levels were measured, and the relationship between IgG levels and parameters suggestive of disease severity in these patients were explored. Patients were placed into one of three groups based on urine protein levels. Results. Urine IgG levels in most patients (99%) increased, while serum IgG in 74% patients decreased. Higher urine and lower serum IgG levels were associated with a more advanced-stage disease, based on the pathology assessment. Urine IgG positively correlated with 24-h urinary albumin, urinary alpha 1- and 2-microglobulin, serum anti-phospholipase A2 receptor (PLA2R) antibody, and erythrocyte sedimentation rate (ESR). However, urine IgG negatively correlated with serum albumin and the estimated glomerular filtration rate (eGFR). Serum IgG positively correlated with serum albumin, but negatively correlated with 24-h urine protein, albumin, urinary a1- and 2-microglobulin, and anti-PLA2R antibody levels. Multivariate regression analyses showed that the 24-h urine protein, urinary alpha 1-microglobulin, eGFR, and ESR were associated with urine IgG levels, independent of age, sex, urinary albumin, urinary 2-microglobulin, or serum anti-PLA2R antibody levels. In addition, serum albumin and age were significant determinants of serum IgG levels. Conclusions. Serum and urine IgG levels are significantly associated with disease severity in patients with IMN and can be useful indicators of IMN incidence.