The significance of polyclonal antibodies for demonstration of hormone expression in pituitary adenomas was compared with the significance of monoclonal antibodies (MAbs). Adenomas were classified by light microscopic structures in paraffin- and epon-embedded sections, by immunostaining for all pituitary hormones, and in part by electron microscopy. In the first series, 166 adenomas were studied with polyclonal antibodies during 1990 and 1991. In the second series, 572 adenomas were immunostained with MAbs during 1999 and 2000. In the first series, a very typical type-specific hormone expression was demonstrable in 63.2% of adenomas in contrast to 91.1% in the second series. Consequently, in the first series 36.7% of adenomas expressed additional or atypical hormones in >10% of adenoma cells, whereas in the second series only 8.9% contained additional hormones. We conclude that MAbs show a clearer cutoff in immunostainings of pituitary hormones and therefore should be preferred for adenoma classification.