Large amounts of antigen may produce falsely low values in immunoradiometric assays due to the so-called high dose, hook effect. The study was designed to identify the clinical and biochemical features of patients with pituitary macroadenomas in whom a high dose PRL hook effect was documented.The clinical and biochemical features of four patients with the high dose PRL hook effect were compared with those of 54 patients with pituitary non-functioning adenomas and 11 with macroprolactinomas who underwent transsphenoidal microsurgery between 1989 and 1994.The presence of the high dose PRL hook effect was confirmed by an increase in the initial PRL concentration when the immunoradiometric assay was processed after dilutions of the serum. This phenomenon was observed in 5.8% (4/69) of patients with pituitary macroadenomas. Undiluted median (range) PRL levels were 9140 (1530-83850), 1530 (162-3210) and 2110 mU/l (1470-45,000) in patients with macroprolactinoma, non-functioning macroadenoma and the hook effect, respectively. In patients with the hook effect, the median PRL levels increased to 384,720 (317,520-950,000) mU/l when the assay was performed after serum dilution. The proportion of males was 9.9% (1/11) in the macroprolactinoma group, 46.3% (25/54) in the non-functioning macroadenoma group and 100% (4/4) in patients with the hook effect. Patients with prolactinoma and non-functioning adenoma had mean tumour sizes of 20 +/- 9 and 27 +/- 11 mm (SD), respectively, while in the hook effect group it was 51 +/- 10 mm.This study suggests that the high dose PRL hook effect is observed particularly in patients with very large tumours. The immunoradiometric PRL assay must be performed with serum dilution in order to overcome the high dose PRL hook effect in all new patients with pituitary macroadenomas who may have a prolactinoma.