Surgical management of Graves' disease (GD) is changing from subtotal to total thyroidectomy because the latter eliminates the risk of recurrence. However, to preserve thyroid function in a euthyroid state, subtotal thyroidectomy is still performed for GD in non-Western countries. Therefore, we designed a study to investigate the long-term outcomes in GD patients after subtotal thyroidectomy and the correlation between remnant weight and postoperative thyroid function. This was a retrospective cohort observation study. Between January 2005 and December 2011, 415 consecutive GD patients treated by subtotal thyroidectomy were enrolled. All data were collected from 385 patients who underwent bilateral subtotal thyroidectomy and 57 patients who underwent the Hartley-Dunhill operation. The median postoperative follow-up time was 72 months (range 12-144 months). The mean weight of the preserved thyroid remnant was 5.1 g. Persistent or recurrent hyperthyroidism was observed in 119 (28.7%) patients. The median time of recurrence was 36 months (range 12-120 months). Hypothyroidism developed in over 50% of patients. A euthyroid state was achieved in only 19.3% of patients, and the rate did not increase significantly as remnant weight increased. Based on a Cox regression analysis, the remnant weight is an independent risk factor for persistent or recurrent hyperthyroidism (hazard ratio: 1.323, 95% confidence interval: 1.198-1.461, P < 0.001). Subtotal thyroidectomy with the intent to maintain a euthyroid state is not an optimal surgical strategy for the definitive treatment of GD because the persistence or recurrence rate is high and the euthyroid rate is lower than expected.