During a two-year period, we evaluated 14 patients with erythrocytosis. Carboxyhemoglobinemia secondary to cigarette or cigar smoking or both caused elevated hematocrit values in ten of the patients, and in all 14 either red cell volume was increased or plasma volume was decreased. There was no correlation between the number of cigarettes or cigars reportedly smoked and either the degree of carboxyhemoglobinemia or the hematocrit value, In three patients who stopped smoking permanently, hematocrit values decreased from a mean of 56% to 46%. Even though the high hematocrit value reversed when these patients stopped smoking, we met with limited success in persuading the others to do so.