Although unilateral Graves` ophthalmopathy is frequently seen, Graves` disease usually presents as a diffuse hyperthyroid goiter, involving both lobes of the gland. Unilateral Graves` disease is a rare disease variant that can occur in either lobe of the thyroid gland. We report here the case of a 39-year old man who presented with unilateral goiter and clinical and laboratory evidence for hyperthyroidism. I-131 radioisotope uptake was elevated at 45.8%, and scintigraphy revealed that uptake of the radioisotope was unilateral increased in the right lobe of the thyroid gland, whereas the uptake in the left lobe did not differ from the uptake in normal control. Ultrasonography of the thyroid gland revealed a inflammatory change and diffuse enlargement in Rt lobe; Doppler investigation of the right lobe showed hypervascularity classically seen in Graves` disease. Serum antibodies to TSH-receptor and thyroid peroxidase were increased. Consequently, hyperthyroidism of Graves` disease with the involvement of only right lobe of thyroid gland was diagnosed. The patient was treated with methimazole. Although the unilateral Graves` disease has occasionally been described in the literature, clinician should be aware that Graves` disease can present unilaterally in either lobe of the thyroid gland.