This study investigated the relationship between peripheral blood B lymphocytes, regulatory T-cells and T lymphocyte subsets, the distribution of B lymphocytes in the kidney, and the pathogenesis of idiopathic membranous nephropathy (IMN). Lymphocyte subsets were measured using flow cytometry in 66 patients with clinically-confirmed IMN and in 40 healthy control subjects. Compared with healthy subjects, the number of peripheral blood B lymphocytes was significantly increased in IMN patients and that of regulatory T-cells was significantly decreased, accompanied by an increased CD4 + /CD8 + T-cell ratio. There was no relationship between the number of peripheral blood B lymphocytes and markers of kidney function. Although the number of infiltrating B lymphocytes in the kidney of IMN patients was higher, there was no relationship with the number of peripheral blood B lymphocytes. In conclusion, there was no relationship between peripheral blood B lymphocytes and disease activity, suggesting that peripheral blood B lymphocytes are not a biomarker of disease activity and therapeutic efficacy in IMN.