We present a 75-year-old Japanese man with essential thrombocytosis presenting nephrotic syndrome. Proteinuria developed soon after the patient was given a diagnosis of essential thrombocytosis and, 4 years later, it increased to a nephrotic range. Renal biopsy revealed one third of the obtained glomeruli totally sclerotic and the other glomeruli showed a marked thickening of the glomerular basement membrane and mild mesangial proliferation. Remarkable widening of the subendothelial space was evident on electron microscopy. Increased expression of platelet derived growth factor receptor β was detected in the mesangium and interstitium by immunohistochemistry. Abnormal platelet activation in myeloproliferative disease has been shown to contribute in glomerulosclerosis by releasing various growth factors and cytokines including PDGF. Considering his clinical course and the pathological findings, the probable risk factor for developing severe endothelial damage and glomerulosclerosis is due to the persistence of high platelet count and platelet abnormality. © Società Italiana di Nefrologia.