Cytologic criteria for diagnosing parathyroid adenomas and hyperplasia have not been elucidated. This report will present the cytomorphology of parathyroid tissue in the spectrum of normal, hyperplastic, and neoplastic states. We examined clinical needle aspirates from three patients with parathyroid lesions as well as 15 needle aspirations or touch preparations from surgical specimens. The series included three with hyperparathyroidism secondary to chronic renal failure, four with primary chief cell hyperplasia, nine with typical adenomas, and two with atypical adenomas. Normal parathyroid glands were also studied. Most parathyroid lesions could be distinguished from thyroid lesions by frequent thick fragments of cohesive cells with frayed edges and typical nuclear features. In general, the nuclei were small (6-8 mu) and round with regular coarse granular chromatin. Occasional larger nuclei (10-30 mu) with similar chromatin were observed in some cases. Oxyphil cells with abundant granular cytoplasm were present in some cases; one case with mostly oxyphil cells was difficult to differentiate from a thyroid neoplasm. Pseudocolloid was found in seven cases. Criteria for the differentiation between parathyroid hyperplasia and adenomas were not discovered. In addition, the cytology and differential diagnosis of atypical parathyroid adenomas are described and illustrated.