Monomeric prolactin (PRL) of molecular weight 23 kDa constitutes up to 95% of adult serum PRL. Macroprolactin is a large antigen-antibody complex of molecular weight greater than 100 kDa and constitutes less than 1% of circulating PRL. Thus, in most cases, hyperprolactinaemia is usually a result of high levels of monomeric PRL, which may be due to excess production as with a prolactinoma or due to disinhibition by compression of the pituitary stalk. Nevertheless, it must be noted that macroprolactinaemia may be a cause of hyperprolactinaemia in some individuals, which may have no association with any pathology. The presence of macroprolactin should always be suspected when a patient's clinical history and/or radiological data are incompatible with his/her PRL value. Thus, it may be useful to screen all patients with high sera PRL levels in order to prevent unnecessary investigations into the cause for hyperprolactinaemia. This has recently been facilitated with the advent of simple laboratory tests such as the polyethyleneglycol precipitation method, although gel filtration chromatography remains the gold standard. It is hoped that macroprolactinaemia is included in the differential diagnosis of hyperprolactinaemia.