We describe two patients, a father and a son, presenting with erythrocytosis. Evaluation showed no pulmonal or cardial disorders. Owing to an elevated erythropoietin level after phlebotomy, a physiological secondary polycythaemia was suspected. A haemoglobin electrophoresis showed that our patients have a haemoglobinopathy with high affinity for oxygen, called Hb-Malmö (exon 3: c.294 C>G p.His98Gln). Hb-Malmö is a congenital disorder located on a gene at chromosome 11, in the B-chain on codon 97, decoding the α-subunit and β-subunit of the haemoglobin. Through a mutation (CAC→CAG), histidine is replaced by glutamine. The mutation causes a disorder in the connection between the α1-subunit and β2-subunit of the haemoglobin structure. These connections are important sites for binding oxygen. Mutated haemoglobin has a preference for an oxygenated status, which implicates that there is an increased binding and decreased release of oxygen. To compensate, there will be an erythrocytosis to transport sufficient oxygen to the peripheral tissues.