BACKGROUND A parathyroid adenoma demonstrating intranuclear holes on aspiration cytology prompted a review of parathyroid neoplasms to determine the frequency of this phenomenon. METHODS Aspiration cytology slides from 30 parathyroid adenomas and 1 parathyroid carcinoma were reviewed. In addition, histologic slides from 136 parathyroid adenomas and 7 parathyroid carcinomas were reviewed. Twenty-two cases had both cytologic and histologic slides available for review. The presence and approximate frequency of intranuclear holes were recorded. RESULTS On cytologic smears, intranuclear holes were found in 3 of the 31 cases reviewed (2 adenomas and 1 carcinoma). The holes were frequent in only one adenoma (the index case) and moderate-rare in the other two cases. Review of tissue slides showed holes in 9 of 136 adenomas and 3 of 7 carcinomas; 2 adenomas and 2 carcinomas had frequent holes, whereas the remainder had holes infrequently. CONCLUSIONS Intranuclear holes do occur in parathyroid neoplasia, in rare cases with considerable frequency. When intranuclear holes are observed in aspiration cytology specimens from the thyroid region, various thyroid and parathyroid conditions must be considered.