Anaphylaxis is a severe and life-threatening systemic reaction. Despite the extensive evaluation to determine the cause, 30%-60% of cases of anaphylaxis in adults remain idiopathic. Recently, omalizumab treatment has been postulated to treat refractory idiopathic anaphylaxis. We report a case of idiopathic anaphylaxis treated with omalizumab and investigated its pharmacological mechanism. A 66-year-old female presented to our clinic with recurrent anaphylaxis. She suffered from anaphylaxis 2-3 times a month for 6 months. She had past medical history of nonallergic bronchial asthma. History was carefully undertaken and anaphylaxis was not related to any specific foods, drugs, exercise, and insect bites. Serum specific IgE antibodies to common food allergens showed negative results. Oral provocation tests to food additives revealed to be negative. To screen systemic mastocytosis and mast cell activating syndrome, baseline tryptase level was checked, and it was within normal range. From comprehensive evaluation, she was diagnosed as having idiopathic anaphylaxis. She could not tolerate oral medications due to gastrointestinal discomfort, therefore, omalizumab treatment (150 mg, monthly) was started. After 6 months of treatment, anaphylaxis did not occur with complete remission status. To evaluate the pharmacological mechanism of omalizumab treatment, basophil histamine releasability test was performed. Histamine releasability induced by anti-IgE did not change after 6 months of treatment, while that induced by calcium inophore decreased. Omalizumab treatment can induce remission or favorable effects on idiopathic anaphylaxis, which may be derived from increased threshold of mast cell degranulation. Long-term studies in a larger cohort will be needed to confirm its efficacy.