The Royal Sun medicinal mushroom, Agaricus brasiliensis, is used as a natural health product. In Japan, however, the quality control of some of these mushroom products has been viewed as a safety problem. Focusing on the quality control of A. brasiliensis KA21, we have performed several safety studies. To date, we have established evidence that this mushroom can be used safely as an immunostimulant and to mediate biochemical parameters associated with obesity or diabetes. Furthermore, to improve the manufacturing process of this mushroom, we have studied the relationship between its pharmaceutical actions and the conditions of its cultivation and thermal management. The purpose of this review is to report the findings of basic and clinical studies of the fruit body of A. brasiliensis KA21.