Arginine vasopressin (AVP) is released upon osmotic stimulation or hypovolemia in order to maintain water balance. A recent study showed a role of AVP in haematopoiesis by stimulating red blood cell precursors, suggesting a higher risk of anaemia in patientis with AVP deficiency. The objective was to explore the effect of low AVP levels in patients with central diabetes insipidus (cDI) and primary polydipsia (PP) on haemoglobin and the prevalence of anaemia.164 patients with either cDI (70, 43%) or PP (94, 57%) and 30 healthy volunteers from two prospective diagnostic studies performed in Switzerland, Germany and Brazil were studied. A standardized clinical and biochemical (e.g. copeptin, full blood count) assessment was performed. Haemoglobin and haematocrit levels and prevalence of anaemia (defined as haemoglobin values of <120 g/L in women and <130 g/L in men) were analyzed.Mean copeptin values were 2.63pmol/L (±1.08) and 3.91pmol/L (±4.28) in patients with cDI and PP and 24.76pmol/L (±5.75) in healthy volunteers, p=0.02. The prevalence of anaemia was low in all participants with 7.1%, 2.2% and 10% in cDI, PP and in healthy volunteers, p=0.15. Mean haemoglobin values were similar in all groups: 139g/L (±15.85), 140g/L (±13.16) and 139g/L (±13.05) in patients with cDI, PP and healthy volunteers, p=0.90, as were mean haematocrit values with 41% in all groups (p=0.85).Chronic low AVP levels in patients with cDI and PP do not affect haemoglobin levels and prevalence of anaemia.