Four cases of anaplastic carcinoma of the thyroid were diagnosed in 1,519 thyroid aspirates. All patients had unilateral or diffuse nodular enlargement of the thyroid. One patient had a concurrent grade 1 mucinous carcinoma of the ovary. Cytologically, the neoplasms were characterized by pleomorphic giant or spindle cells with multilobulated, single or multiple nuclei. The chromatin pattern was clumped with prominent parachromatin clearing, and there were multiple prominent nucleoli. Associated papillary and follicular structures were recognized in two aspirates. All cytologic preparations were considered diagnostic of malignancy. Examination of histologic material revealed the same elements identified cytologically on the aspiration smears. Three patients died shortly after diagnosis due to massive local tumor growth in the neck. One patient, who received radiation therapy, had a prolonged survival of 30 mo, but died then of local neck recurrence. The cytologic features of anaplastic carcinoma parallel their histologic appearance. Histologic confirmation of cytologically diagnosed anaplastic carcinoma is not necessary.