Since macrocytosis was observed in a high percentage of our renal transplant patients, a follow-up study was carried out on 36 patients with determination of the mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and haemoglobin concentration (MCHC). These parameters were determined monthly for periods of up to two years (16 cases) following transplantation. There was a rise in both MCV and MCH within one month. Macrooperative MCHC, however, remained normal. Anaemia was absent or mild. The bone marrow showed striking megaloblastic changes with nuclear-cytoplasmatic dissociation in both red and white cell precursors. Vitamin B 12 absorption tests were normal in all 8 patients investigated. Macrocytosis was not detectable in two patients with a low serum iron concentration. Macrocytosis disappeared in 4 patients with chronic rejection when progressive renal failure developed. It is concluded that azathioprine therapy induces macrocytosis in renal transplant patients provided renal function is satisfactory.