summary Delayed haemolytic transfusion reactions (DHTRs) are relatively common following blood transfusions. In the UK, DHTRs were responsible for 10.2% of all serious transfusion‐related hazards between 1996 and 2003. In most cases, there is minor or no morbidity, and the reaction may go unnoticed. However, in some cases, a large fall in haemoglobin level and major morbidity may occur. Even in such serious cases, the true cause of the patient’s clinical deterioration may be unrecognized if serological investigation is not carried out immediately because of the often transient nature of the antibodies involved. Failure to make the correct diagnosis will lead to inappropriate treatment and expose the patient to risk of further serious transfusion reactions. We describe a case of DHTR caused by anti‐Jkb antibodies, which illustrates the difficulty in diagnosing this common but under‐recognized reaction and shows the transient nature of the antibody involved, which became undetectable within 4 weeks.