Considerable erythrocytosis was noted in six patients with parenchymal renal disease. Kidney biopsies performed in four patients revealed focal sclerosing glomerulonephritis in two and chronic vascular changes with interstitial scarring in two others. The remaining two patients had heavy proteinuria suggestive of glomerular involvement. No known causes of secondary erythrocytosis were found. The findings in these patients as a group did not fit the criteria for polycythemia vera. Three patients who progressed to chronic renal failure had diminution of erythrocytosis. It is concluded that parenchymal renal disease, primarily of a glomerular or vascular type may provoke erythrocytosis. Explanations of the pathogenetic mechanism for this association remain speculative.