Medullary thyroid carcinoma typically presents as dyscohesive plasmacytoid, spindled, or polygonal cells on fine-needle aspiration smears. We recently encountered a case of sporadic medullary thyroid carcinoma that presented as a hypercellular aspirate composed of cohesive aggregates of rectangle-shaped cells. The case was mistakenly reported as a hypercellular follicular neoplasm on cytology. Subsequent thyroidectomy revealed medullary carcinoma. We draw attention to this distinctive rectangular cell type as an additional morphology for medullary thyroid carcinoma. Diagn. Cytopathol. 2009. © 2009 Wiley-Liss, Inc.