Objectives：There are various reports for incidence and risk factors of hypothyroidism after hemithyroidectomy for benign thyroid disease. The reported incidence rate varies from 5.0 to 41.9%. Moreover, there are few reports about this in Korea. So, our objective is to identify the incidence and possible factors contributing to hypothyroidism after hemithyroidectomy in Korea. Subjectives and Methods：We reviewed medical records of patients who underwent hemithyroidectomy between 1999 and 2007 years. We enrolled 80 patients. The incidence of postoperative hypothyroidism was based on thyrotropin values and clinical symptoms. And all patients were evaluated for age, gender, serum thyrotropin(TSH) levels, serum free T4, weight of resected thyroid tissue and associated thyroiditis. Results：Nine of 80 patients(11.3%) became biochemically hypothyroid postoperatively. Six of 9 hypothyroid patients were diagnosed within 2 months of operation. The mean preoperative TSH level was $2.12{pm}0.96mIU/L$ in hypothyroid and significantly different from $1.33{pm}0.77mIU/L$ in euthyroid patients(p<0.05). There were no significant differences in age, gender, weight of resected thyroid tissue, preoperative free T4 and associated thyroiditis. Conclusion：A minority of patients became hypothyroid after hemithyroidectomy. Hypothyroidism following hemithyroidectomy occurred in patients with higher preoperative TSH levels.. The screening test for postoperative hypothyroidism should be performed from month to year after operation.