Erythrocytosis or polycythemia is defined as an increase in red blood cell concentration above the age- and sex-specific normal levels. Unlike anemia, which is very common in patients with chronic kidney disease (CKD), erythrocytosis is less frequent but requires specific understanding by health care professionals in order to provide the best care. Erythrocytosis, especially when undiagnosed and untreated, can lead to serious thrombotic events and higher mortality. Classic causes of erythrocytosis associated with CKD include cystic kidney diseases, kidney or other erythropoietin-secreting neoplasms, high-altitude renal syndrome, overdosage of erythropoietin-stimulating agents, androgen therapy, heavy smoking, chronic lung disease, obstructive sleep apnea, IgA nephropathy, post–kidney transplant erythrocytosis, renal artery stenosis, and congenital etiologies.