Anti-Leu-7 (HNK-1, CD 57) antibody, a marker for natural killer lymphocytes, was employed by Ghali et al. (Hum Pathol 1992;23: 21-25) to study surgically resected formalin-fixed, paraffin-embedded thyroid lesions. They demonstrated strong immunoreactivity of this antibody with thyroid carcinomas, both follicular and papillary, and only occasional weak immunoreactivity with colloid goiters and follicular adenomas. We studied cytologic specimens (primarily fine-needle aspiration biopsy specimens) from 44 thyroid lesions, including 10 follicular carcinomas, 14 follicular adenomas, seven adenomatous nodules, six papillary carcinomas, and seven cases of Hashimoto's thyroiditis. All follicular carcinomas exhibited immunoreactivity to anti-Leu-7 antibody, usually of a moderate to strong degree (9/10); however, six of 14 follicular adenomas yielded similar results. The patterns of immunoreactivity in the other lesions were similar to those previously described (Ghali et al., Hum Pathol 1992;23:21-25). It does not appear that anti-Leu-7 antibody can be used as a specific marker of malignancy in the cytologic assessment of follicular neoplasms of the thyroid.