We report four unusual cases of hyalinizing trabecular thyroid tumors, three of which displayed capsular and/or blood vessel invasion and were classified as minimally invasive carcinomas. These three patients were all euthyroid women aged 18, 39, and 62 years. The youngest patient also had familial polyposis, while the oldest patient was being followed for Hashimoto's thyroiditis. All three lesions were solitary nodules, were "cold" by radioactive iodine thyroid scan, and ranged from 2.5 to 4.0 cm in diameter. The tumors were encapsulated and had growth patterns and cytologic features similar to those described for hyalinizing trabecular adenomas. In fact, the only difference between these minimally invasive hyalinizing trabecular carcinomas and the corresponding adenomas was the presence of capsular invasion in all three of the cases and vascular invasion in two. All three patients are free of recurrent or metastatic disease at 1 to 2 years' follow-up. The fourth patient, a 48-year-old woman, had a micropapillary carcinoma within a hyalinizing trabecular adenoma. She is also free of disease 5 years following excision of the involved lobe. These findings suggest that hyalinizing trabecular thyroid tumors have a malignant counterpart, similar to conventional follicular adenomas, and that papillary thyroid carcinomas may arise within hyalinizing trabecular thyroid tumors.