We investigated endothelin-1 mRNA expression in peripheral blood monocytes from 40 patients with IgA nephropathy, 40 with other glomerulonephritides and 10 with essential hypertension without renal diseases, and from 30 healthy age-matched controls. 95% of patients with IgA nephropathy had increased endothelin-1 mRNA expression in monocytes but little was detected from the other groups. Endothelin-1 mRNA was positively correlated with urinary protein excretion (95% CI 0.835-0.952) and histopathological findings (0.796-0.939). In 15 patients with IgA nephropathy endothelin-1 mRNA values and proteinuria decreased gradually after treatment. Endothelin-1 mRNA-expressing activated monocytes may be associated with the progression of IgA nephropathy.