Prolactin (PRL) is an important immunoregulatory hormone secreted by the anterior pituitary gland. Hyperprolactinaemia has been implicated in the pathogenesis of systemic lupus erythematosus (SLE). However, clinical studies regarding the PRL level and lupus disease activity have yielded contradictory results. The aim of our present study was, therefore, to re-evaluate the association of PRL level and disease activity in SLE by analysing a larger patient cohort and following them up serially. Seventy-two consecutive SLE patients were recruited and the serum PRL level was measured at each visit. Our results showed that hyperprolactinaemia (> 500 mIU/l) occurred in 35% (25/72) of the patients. A total of 72% (18/25) of the hyperprolactinaemic patients had mild elevation (arbitrarily defined as 500-800 mIU/l) of the level only. No correlation could be found between the PRL level and various clinical and serological parameters of lupus disease activity. On serial follow-up of 44 patients, again no correlation between PRL and disease activity could be demonstrated. We conclude that hyperprolactinaemia occurs in some patients with SLE, but the serum level of PRL does not correlate with clinical or serological disease activity and is not a reliable marker for disease monitoring. The mechanism and pathoaetiological and clinical significance of hyperprolactinaemia in a small subset of SLE patients remain unclear and a longer follow-up is necessary.