The aim of this study was to investigate the expression of Ki-67, bcl-2 and p53 in parathyroid adenomas and their residual rim of normal parathyroid tissue. Specimens from 26 parathyroid adenomas were studied by immunohistochemical analysis for Ki-67, bcl-2 and p53 expression. Positive findings were noted for p53 in 4 (15%) adenomas and none of the residual rims of normal parathyroid tissue (p = 0.055); for Ki-67 in 15 (56%) adenomas and none of the residual rims of normal parathyroid tissue (p = 0.00002); and for bcl-2 in 19 (73%) adenomas and 8 (31%) residual rims of normal parathyroid tissue (p < 0.01). The high rate of Ki-67 expression may indicate susceptibility of parathyroid adenomas to clonal proliferation. The weak immunoreactive expression of p53, combined with a relatively strong expression of bcl-2, may contribute to the characteristic slow progression of these tumors.