Too many RBCs. Often presents asymptomatically, can have issues with clotting, hypermetabolism (gout, kidney stones, sweating, itching), hyperviscosity, vasomotor symptoms (erythromyalgia), bleeding, splenomegaly. Erythrocytosis most common cause - chronic hypoxia (smoking, sleep apnea), ectopic EPO production, high oxygen affinity hemoglobinopathies. Testosterone use is also a common cause of erythrocytosis.