OBJECTIVE: To analyze the effect of needling acupoints (bilateral vs unilateral) with De Qi using data collected from 501 primary dysmenorrhea (PD) patients participating in multi-center, randomized, controlled trail. METHODS: De Qi was defined as at least one of the feelings in soreness, numbness, fullness or heaviness at the acupoints when stimulated with needles. The 501 patients were grouped in 3 groups in terms of De Qi or not De Qi in one side (unilateral) or both sides (bilateral) of the body: bilateral De Qi group, unilateral De Qi group, and non-De Qi group. The abdominal pains were measured using visual analog scale (VAS). RESULTS: In 501 patients, 472 acquired De Qi at unilateral acupoints, 24 De Qi at bilateral acupoint, and 5 had no De Qi at any acupoint. The data of non-De Qi group was excluded as the sample was less than 5% of that in the bilateral De Qi group. There was significant difference in the VAS before and after treatment between unilateral and bilateral De Qi group (P < 0.01). After stratified by acupoints, for the patients needled at Sanyinjiao (SP 6) and Xuanzhong (GB 39), VAS scores in the bilateral De Qi group were larger than those in the unilateral De Qi group (P < 0.05). CONCLUSION: Bilateral De Qi was possibly superior to unilateral De Qi in enhancing the immediate analgesic effect of needling the acupoints, but no statistical significance was observed on the patients of needling at non acupoint, which preliminarily suggested this immediate analgesic effect was perhaps along meridians. (C) 2016 JTCM. All rights reserved.