A 44-year-old woman presented with hyperthyroidism and a nodule in her thyroid. 131I-scintigraphy of the gland showed concentration of the tracer within the nodule and suppressed activity in the rest of the gland. Histology of the resected thyroid proved the nodule to be a papillary carcinoma. The presence of carcinoma within an autonomous hyperactive thyroid in a patient with hyperthyroidism has been reported only rarely, and to the authors' knowledge such a case has never been reported in Great Britain. Furthermore, unlike previously reported cases, the carcinoma was confined solely to the nodule.