This report describes a 50-year-old woman with coexisting Basedow's disease and adult-onset Still's disease (AOSD) that relapsed simultaneously. She was diagnosed with Basedow's disease in 1999, and treatment with antithyroid agents was started. However, the treatment was soon stopped because of severe side effects. A partial thyroidectomy was performed and the thyroid function stayed well-controlled after the surgery. In August 2007, she was admitted to our hospital with fever, a sore throat, skin rashes, arthritis and leukocytosis, and was diagnosed with AOSD. At the same time, her laboratory data revealed decreased serum TSH and elevated serum free T4, suggesting a relapse of Basedow's disease. After initiation of steroid pulse therapy accompanied by oral prednisolone, both diseases improved significantly. Prednisolone was gradually reduced, and the disease activity remained in remission. Immediately after prednisolone reached 3 mg/day in November 2009, both diseases relapsed. Prednisolone was increased to 30 mg/day, and the diseases became well-controlled again. In this case, Basedow's disease was aggravated when AOSD was in the active stage. Literature searches revealed five previously reported cases with coexisting Basedow's disease and AOSD. In four of the six cases, including our case, both diseases were activated simultaneously. AOSD in the active stage is known to cause hypercytokinemia and immunological derangement. Our case indicated that the pathogenesis of AOSD might lead to relapse of coexisting Basedow's disease.