Sunitinib has recently become a standard treatment for metastatic renal cell carcinoma. However, various adverse events have been reported. We present the first case of clinically evident adrenal insufficiency during sunitinib therapy. A 72-year-old man began sunitinib therapy for bilateral lung and adrenal metastases of renal cell carcinoma. His adrenocorticotrophic hormone level was 93.6 pg/ml (7.2-63.3 pg/ml) before sunitinib treatment, indicating that subclinical adrenal insufficiency already existed. Fatigue, which is a frequently seen adverse effect of sunitinib treatment, emerged acutely on Day 24 of the second cycle. Adrenocorticotrophic hormone and free T4 were high and thyroid-stimulating hormone was suppressed. Under the clinical diagnosis of acute adrenal insufficiency with thyrotoxicosis, a low dose of steroid was administered. Fatigue was completely ameliorated by the following morning, although free T4 was still high and thyroid-stimulating hormone was still low. Therefore, hypermetabolism due to thyrotoxicosis unmasked adrenal insufficiency in our case. Physicians should be aware of this rare but potentially fatal complication when severe acute fatigue develops in patients with subclinical adrenal insufficiency.