Background: In the vast majority of patients with chronic urticaria , the etiology is unknown. It has been noted that in one third of such patients it has an autoimmune base. Autologous serum skin test (ASST) is currently the best in vivo test for the detection of such patients. Our aim was to identify the prevalence of positive ASST among patients with chronic idiopathic urticaria (CIU) and to compare features of ASST + and ASST – groups. Methods: this study was done on 79 patients with CIU. Samples of 0.05ml of autologous serum and normal saline were separately injected intradermaly into the volar aspect of right and left forearm respectively. ASST Was deemed to be positive if a serum induced weal had a diameter bigger than a sailin induced response by < 1.5 mm was seen at 30min. Also we compare the clinical and laboratory parameters of ASST+ and ASST - groups.Results: Out of 79 patients with CIU, 39 patients (39%) had positive ASST and it was found that the frequency of attacks was higher in these patients (P=0.04). Conclusion: Significant percent of patients with CIU showed positive response to ASST which is most likely an autoimmune source for them. ASST can be used as a simple and cost - effective test for the detection of these patients, which has proven to be a therapeutic challenge to the treating physician.