Proteinuria (urine protein/creatinine ratio, 13.6) resolved after control of primary erythrocytosis in a dog. Hydroxyurea and doxorubicin administration and phlebotomy were used initially to manage erythrocytosis. Remission was maintained for approximately 2 years. Glomerulonephropathy, characterized by absence of routine histologic or immunofluorescent changes and ultrastructural evidence of basement membrane deterioration and podocyte fusion, was documented. These lesions may have been a result of hypoxia and/or hyperviscosity secondary to erythrocytosis.