In a 34-year-old woman with primary subfertility, a strongly increased serum concentration of prolactin was found in combination with normal levels of oestradiol, which is an indication for the presence of prolactin forms without clinical effect. She appeared to have macroprolactinaemia, i.e. the presence of circulating large forms of prolactin (up to > 100 kDa), which may be detected by the immunoassays currently used. They give no clinical signs or symptoms. Diagnosing macroprolactinaemia means that further diagnostic tests for hypopituitarism abnormalities using MRI need not be carried out and unjustified treatment of otherwise healthy persons may be prevented.