The literature describes a number of vascular alterations in the thyroid gland and its tumors, the majority of which are related to fine-needle aspiration (FNA). These alterations include pseudoangiosarcomatous changes (Masson's lesion), pseudoinvasion in capsular veins, and endothelial proliferations in the needle tract. We present three cases of a unique intravascular endothelial proliferation in the capsular vessels of follicular and/or Hürthle cell thyroid neoplasms (two angioinvasive Hürthle cell carcinomas and one angioinvasive follicular carcinoma), which we think is unrelated to FNA. The lesion consists of spindle cells with plump nuclei, with focal nesting imparting an epithelioid pattern to the lesion. Focally red blood cells percolate between and mix with the spindle cells, recapitulating superficially a Kaposi's like appearance, but we identified no mitotic figures. Only one patient had a preoperative FNA; in that case, the lesions were spatially separate from the needle tract and the usual post-FNA reactive changes. The lesional cells were positive for Factor VIII and CD 31 and negative for cytokeratins and thyroglobulin immunostains. This immunopanel was helpful in distinguishing these lesions from true angioinvasion by thyroid tumor. To date, the etiology of these vascular lesions is unknown, but we postulate that certain follicular-derived thyroid tumors might elaborate angiogenic mediators, which in a cell culture system of follicular thyroid carcinoma can trigger endothelial proliferations.