Five patients, one woman and four men, aged 24 to 54 years, developed abnormally high hematocrits between 4 and 22 months after renal transplantation. In four patients, red cell mass was above normal and plasma volume below normal. In one patient, both red cell mass and plasma volume were below normal. The cause of the deranged plasma volume was not evident. Decreased plasma volume may either contribute to or may, in rare instances, be the only cause of apparent erythrocytosis in renal transplant recipients.