The optimal surgical management of well-differentiated thyroid cancer (DTC) remains a controversial topic. Preoperative and peroperative investigations quite frequently fail to detect thyroid cancer in cold nodules, and only postoperative histological examination reveals malignancy. In these cases many physicians perform a completion thyroidectomy. Others recommend a conservative approach with close follow-up because of the increased risk of complications after re-operation. In our department, routine management includes completion thyroidectomy once the histopathological report concludes that there is carcinoma, except in cases of papillary carcinoma measuring less than 1 cm. The aim of our study was to determine the incidence of contralateral malignancy and of complications after completion thyroidectomy. We reviewed the records of 29 patients--25 women and 4 men-- who all underwent completion thyroidectomy because of an unexpected diagnosis of DTC. Residual malignancy was found in 12 patients (41.4%) after completion thyroidectomy. In ten patients (34.5%) the malignancy was localised in the contralateral lobe and two patients (6.9%) had lymph node metastases. Postoperative transient hypocalcaemia (< 8.0 mg/dl) occurred in five patients (17.2%) and permanent hypocalcaemia (lasting longer than 6 months) was a feature in two patients. One patient suffered transient laryngeal nerve injury occurred in one patient and there were no permanent lesions. In conclusion, we found residual DTC in 41.4% of patients undergoing reintervention. Because of the rather low re-operation rate, we prefer to perform a completion thyroidectomy to remove potential occult malignancy and to allow for postoperative 131I-treatment in all patients with a diagnosis of malignancy in their thyroid lobectomy specimen, with the exception of papillary carcinoma < 1 cm.