A retrospective study on 22 cases of parathyroid adenoma, 9 cases of primary parathyroid hyperplasia and 14 specimens of normal suppressed glandular tissue was undertaken to determine the usefulness of proliferative index (PI) for discriminating adenoma from hyperplasia, as an adjunct to the existing histological criteria. PI was determined by avidin-biotin-complex immunostaining after high temperature microwave antigen retrieval in paraffin sections, using monoclonal MIB-1 antibody which detects paraffin resistant analogue of cell cycle-associated Ki-67 antigen. PI expressed as percentage positive cell nuclei, was 1.36 +/- 0.62 (range 0.04-2.72) in adenoma, 1.17 +/- 0.83 (0.02-1.98) in hyperplasia and 0.03 +/- 0.02 (0.00-0.06) in normal suppressed glandular tissue. While the difference between normal suppressed glandular tissue and adenoma and hyperplasia was significant (P < 0.001), that between adenoma and hyperplasia was not. We conclude that although PI could distinguish between normal suppressed glandular tissue versus glands with primary hyperparathyroidism, it failed as an additional useful parameter for discriminating between adenoma and hyperplasia, both of which have low but similar proliferative activity.