Background: Thyrotoxic storm is a life-threatening condition requiring emergency treatment. The condition arises in thyrotoxic patients who manifest decompensation in multiple organs, often triggered by severe stress. Neither its epidemiological data nor diagnostic criteria have been fully established. We attempted to clarify the clinical and epidemiological characteristics of thyroid storm, initially using nationwide surveys and then formulate diagnostic criteria for thyroid storm. Methods and Subjects: To perform the nationwide survey on thyroid storm, we first developed tentative diagnostic criteria for thyroid storm, mainly based upon the literature (the first edition). The tentative diagnostic criteria, which included the requirement that thyrotoxicosis be present, defined definite and suspected cases based on the prerequisite item (the presence of thyrotoxicosis) and combinations of typical clinical features. We then conducted the initial and second nationwide surveys using these criteria, targeting all hospitals in Japan with eight-layered random extraction. Using the results of the second survey, we analyzed the relationship of the major features of thyroid storm to mortality and to certain other features. Finally, based upon the findings of these surveys, we revised the diagnostic criteria. Results: According to the initial nationwide survey, the number of definite and suspected thyroid storm cases was estimated to be 1,283 ± 105 (95% confidence interval: 1,077 - 1,489) per five years (0.20 persons/100,000 Japanese population/year). Thyroid storm occurred in 0.22% of thyrotoxic patients. The second nationwide survey obtained detailed clinico-epidemiologic features of 282 definite and 74 suspected cases. The mortality rates of definite and suspected cases were 11.0% and 9.5%, respectively. Finally, based on the results of these surveys, we propose a revision of the diagnostic criteria (the second edition). Conclusions: Thyrotoxic storm is still a life-threatening disorder with over 10% mortality in Japan. We newly formulated diagnostic criteria for this disorder.