5-(hydroxymethyl)-2-furaldehyde (5-Hydroxymethyl-2-furancarboxaldehyde, 5‐hydroxymethyl‐2‐furfural) is found in garden onion. It is obtainable from various carbohydrates. 5-Hydroxymethyl-2-furancarboxaldehyde is present in tomatoes, tobacco oil etc. It is a constituent of numerous plant species. 5-Hydroxymethyl-2-furancarboxaldehyde is used as an index of heat treatment and deterioration in food such as tomato paste, honey and fruit juices. Also an indicator of adulteration with acid-converted invert sugars. The heterocyclic aldehyde 5‐hydroxymethyl‐2‐furfural (5HMF, Aes‐103, BAX 555) interacts allosterically with the abnormal form of haemoglobin (Hb), HbS, in red blood cells (RBCs) from patients with sickle cell disease (SCD), thereby increasing oxygen affinity and decreasing HbS polymerization and RBC sickling during hypoxia. 5HMF markedly reduced the deoxygenation-induced dehydration of RBCs whether in response to maintained deoxygenation or to cyclical deoxygenation/re-oxygenation. 5HMF was found to inhibit Psickle, an effect which correlated with its effects on sickling. Deoxygenation-induced activation of the Gardos channel and exposure of phosphatidylserine were also inhibited, probably indirectly via reduced entry of Ca(2+) through the Psickle pathway. Effects of 5HMF on KCC were more modest with a slight inhibition in N-ethylmaleimide (NEM, 1 mm)-treated RBCs and stimulation in RBCs untreated with NEM. These findings support the hypothesis that 5HMF may also be beneficial through effects on RBC ion and water homeostasis. AesRx is developing Aes-103 as an orally bioavailable, chronic therapy for SCD. Aes‐103 is currently in phase II clinical trials in SCD patients in the USA and UK. Aes-103 has received orphan designation from the US Food and Drug Administration and is eligible for orphan status in Europe.