The histologic spectrum of proliferative parathyroid lesions (hyperplasia, adenoma, and carcinoma) often overlap, and differentiation between these lesions may at times be difficult. p27kip1 (p27) is a cyclin-dependent kinase inhibitor that helps regulate the transition from the G1 to the S phase of the cell cycle. Significantly higher levels of p27 expression have been detected in some normal tissues than in their neoplastic counterparts. The authors analyzed a series of parathyroid lesions to determine if expression of this cell cycle protein may be useful in distinguishing between parathyroid hyperplasia, adenomas, and carcinomas. Formalin-fixed paraffin-embedded tissues from randomly selected patients (22 histologically normal parathyroid glands, 33 cases of hyperplasia, 43 adenomas, and 17 carcinomas) were analyzed for expression of p27 by immunostaining. All cases were also immunostained for Ki67 with antibody MIB-1. The distribution of immunoreactivity was analyzed by quantifying the percentage of positive nuclei that was expressed as the labeling index (LI). In situ hybridization (ISH) for p27 mRNA was done using a cRNA probe with 30 of these cases. Normal parathyroid glands had the highest p27 LI (89.6 +/- 1.4), followed by hyperplasia (69.6 +/- 7.5), adenomas (56.8 +/- 3.4), and carcinomas (13.9 +/- 2.6). ISH showed no differences in p27 mRNA, indicating that the expression of the p27 gene was controlled at a posttranslational level in parathyroid tissues. Ki67 expression was significantly higher in carcinomas (LI = 8.4 +/- 1.9) than in adenomas (LI = 2.7 +/- 0.2) and hyperplasia (LI = 3.3 +/- 0.4). These results suggest that both p27 and Ki67 may be helpful in the diagnosis of histologically difficult parathyroid lesions.