Forty-four follicular thyroid neoplasms followed for a minimum of 10 years are presented. The cases were divided into four categories: follicular adenoma (19 cases), encapsulated follicular carcinoma (7 cases), invasive follicular carcinoma (11 cases), and the follicular variant of papillary carcinoma (7 cases). The former two tumor types were distinguished by the presence of capsular invasion in encapsulated follicular carcinoma (which was defined as lacking extension into the surrounding thyroid) and by the frequent thinness of the capsule in follicular adenoma, while the latter two (which both infiltrated adjacent thyroid or extrathyroid tissue) were separated primarily on the basis of the tendency toward vesicular and hypochromatic nuclear staining in the follicular variant of papillary carcinoma, but with consideration of a number of other factors in addition. No patient with follicular adenoma had any further difficulty after removal of the lesion. Metastasis to cervical lymph nodes occurred in five cases of the follicular variant of papillary carcinoma, but there were no distant metastases or deaths due to tumor. By contrast, three patients with encapsulated follicular carcinoma and nine with invasive follicular carcinoma died of tumor, and all of these had distant metastasis (most often to bone). Local recurrence was seen only in invasive follicular carcinoma (six cases).