['Surgical excision has been recommended as a diagnostic test for thyroid nodules ≥ 4 cm, due to the supposedly higher rate of cancer in larger nodules and the higher reported false-negative rates of fine-needle aspiration cytology (FNAC) testing (>10%). The aims of this study are to determine the prevalence of thyroid cancer in nodules ≥ 4 cm, to examine if a relationship between increasing nodule size and malignancy rate was present and to study the accuracy of preoperative FNAC diagnosis.', 'Retrospective analysis of data from patients with thyroid nodules ≥ 4 cm undergoing surgical resection between 1994 and 2008. Malignancy rates, cytology results and indications for surgery were analysed.', 'A total of 223 patients with thyroid nodules ≥ 4 cm underwent thyroid resection between 1994 and 2008. The overall prevalence of thyroid cancer was 7.2% (95% confidence interval (CI): 4.2-11.4%). The malignancy rate did not vary significantly with increasing nodule size. The sensitivity of FNAC was 93.8% (95% CI: 69.8-99.8%), while the specificity of FNAC was 62.2% (95% CI: 54.9-69.2%). The most common indicator for surgery was compression symptoms. A positive FNAC test was the most significant indicator of underlying malignancy, with a likelihood ratio of 2.5 (95% CI: 2.0-3.1).'] Is diagnostic lobectomy routinely required to exclude malignancy in thyroid nodules greater than four centimetres?