The aim of this study was to evaluate the usefulness of DNA flow cytometry to determine tumor nuclear DNA index (DI), and nucleolar organizer region protein counts visualized by the argyrophil (AgNOR) technique, in confirming diagnosis and predicting clinical outcome of patients with parathyroid carcinoma (PC). We reviewed paraffin-embedded tissue sections, from 15 patients (median age 63 years, range 30-68 years) with PC who died of the disease, which were randomly compared with tissue sections from 15 age- and sex-matched patients with parathyroid adenoma (PA). The proliferative activity in parathyroid tumours as detected by DI and AgNOR counts was evaluated in all specimens. Both DI (1.37 +/- 0.33 vs 1.0 +/- 0.1) and AgNOR (3.01 +/- 0.31 vs 1.54 +/- 0.35) counts were higher (P < 0.001) (Student's t-test) in patients with PC than in those with PA. Diploid (DI = 1), aneuploid (DI > 1) and hypoploid (DI < 1) neoplasms were found in 11 (PC = 4, PA = 7), 14 (PC = 11, PA = 3) and five (PC = 0, PA = 5) patients respectively. The average postoperative survival in patients with PC was 46.9 +/- 37.4 months (range 21-146 months). The survivals of patients with aneuploid (n = 11) and diploid (n = 4) PC were 74.0 +/- 58.1 and 34.1 +/- 18.4 months (P=0.21) respectively. There was a significant relationship between DI and AgNOR counts (R=0.69, P < 0.01), but no correlation was found between survival and both DI (Rho = 0.17, P = 0.55) and AgNOR counts (Rho = 0.26, P = 0.35). Moreover, there was no correlation (P = NS) between the main preoperative biochemical parameters and survival. In conclusion, DI and AgNOR are useful in confirming the diagnosis of PC, but they are of little value in predicting the clinical outcome of patients with PC.