OBJECTIVES To determine the prevalence of thyroid cancer in patients with hyperthyroidism. METHODS This is a retrospective observational study using the data of 71 Omani patients with a diagnosis of hyperthyroidism due to Grave's disease, toxic multinodular goiter, and solitary toxic adenoma. These patients underwent thyroidectomy at the Royal Hospital (RH), Muscat, Oman, and were followed up at the National Diabetes and Endocrine Center (NDEC) between 2007 and 2013. The details were collected from the medical records of both the RH and the NDEC. Patients who underwent thyroidectomy for other reasons like non-toxic goiter and hypothyroidism with cancer were excluded from the study. RESULTS Thyroid cancer was identified in 32.8% (n=23) of patients with hyperthyroidism. Half of these patients 52.1% (n=12) had papillary micro-cancer (intra-thyroidal), and 3 patients with Grave's disease (13%) had lymph nodes metastasis (loco-regional infiltration. The cancer preponderance was higher in young (n=21, 91.3%) and female patients (n=18, 73.9%). Most patients with thyroid cancer had abnormal ultrasound neck findings and thyroid scintigraphy (99 mTc uptake). CONCLUSION Many patients with hyperthyroidism in Muscat, Oman, especially those with Grave's disease, show malignancy, and hence a proper initial evaluation of these patients is required as part of long-term management.