Simple morphological identification of a sample as parathyroid gland tissue is not always sufficient for optimal patient treatment. Instead, patients with parathyroid gland lesions that increase the risk of disease relapse should be identified. To assess the possibility of differentiating adenoma from hyperplasia in preoperative material by use of computerized morphometric analysis in order to enable better preoperative work-up in patients with hyperparathyroidism. Samples obtained by US guided fine-needle aspiration biopsy of 67 parathyroid glands without known histopathologic diagnosis were dried and stained by May-Grünwald-Giemsa. Fifty nuclei per sample were analyzed and designated automatically with manual correction using image analyzer and specific software. The area, circumference, convexity, minimal and maximal radius, length and width, and factors of regularity, i.e. form factor and factor of nucleus elongation, were determined for each nucleus. Basic descriptive parameters and measures of variability (variation coefficient and standard deviation) were calculated for each continuous variable. The correlation of continuous variables was analyzed by use of Mann-Whitney test. Computer analysis of cell image classified the cell nuclei into two groups. The area, circumference, minimal radius, convexity and width of the nuclei showed higher values in the hyperplasia group as compared with the adenoma group. Standard deviation also showed higher values of each of the study parameters in the former group. Our own observations and subjective assessment of variations in nucleus size to represent substantial characteristics differentiating parathyroid adenoma and hyperplasia in cytologic smear were quantitatively verified by the use of objective morphometric measurement and should therefore be considered valid parameters on differentiating these two entities.