Chronic idiopathic urticaria (CIU) is a common dermatological disorder which signifi cantly affects the quality of life of patients worldwide. This study investigates the demographic features, laboratory fi ndings and the response to treatment of CIU patients prospectively in a medical center in northern Taiwan. Methods and Results: Thirtythree patients (15 females, 18 males, median age 32.79 years) with CIU were included according to the protocal approved for this study. A laboratory survey, including complete blood cells (CBC) with differential count, erythrocyte sedimentation rate (ESR), biochemistry panels, autoimmune profi les, thyroid function and multiple allergy simultaneous test (MAST) were conducted and analyzed before and after once daily monotherapy with levocetirizine 5mg. In addition, therapeutic effi cacy was also assessed using mean pruritus score (MPS) and the mean number of wheals (MNW) score. The most commonly found reactive agents in the specifi c allergen test were mite DF (18/33 subjects, 54.4%) and mite DP (18/33 subjects, 54.4%). No significant difference was noted before and after levocetirizine monotherapy in terms of CBC, AST, ALT, BUN, creatinine, and ESR. Furthermore, the result of the MAST was also not affected by levocetirizine treatment. Compared to the therapeutic outcome measured on day-8, administration of antihistamine treatment signifi cantly decreased MPS and MNW scores during day-22, day-29, and day-30. Consistently, the mean scores of MPS and MNW were signifi cantly lower on day-36 than on day-8 within the same IgE level. Conclusions: Most of the patients in this study experienced satisfactory clinical improvement after once daily monotherapy with levocetirizine 5mg, whereas the antihistamine treatment did not result in a statistically signifi cant change in the laboratory investigations. This study demonstrates that exhaustive investigations may not be necessary for patients with CIU and should be reserved for individuals following acquisition of a detailed history and physical examination. However, the monotherapy with low-sedative levocetirizine demonstrated a good clinical effi cacy and did not signifi cantly alter the laboratory parameters including the result of MAST. Therefore, it is suggested that MAST be included for checking which allergens should be avoided.