Objective: To investigate how individuals expressed rationales for their beliefs regarding efficacy of acupuncture. Methods: Qualitative data from participants of two different randomized sham-controlled trials, of relaxing (non cancer volunteers of the general population) or antiemetic (patients with cancer undergoing radiotherapy) effects of acupuncture was analyzed. Participants (n = 441) received genuine (n = 120 and n = 100) or sham (n = 121 and n = 100) (telescopic blunt sham-needle) relaxing or antiemetic acupuncture. The participants (n = 428; 97% response rate) expressed their belief regarding the efficacy of acupuncture, and n = 264 delivered qualitative rationales for their belief, analyzed using qualitative content analysis. Results: Of the 428 participants, 35 (8%) believed entirely that the acupuncture was effective, 209 (49%) believed much, 136 (32%) believed moderately, 39 (9%) believed a little, and 9 (2%) did not believe that the acupuncture was effective. Five categories and seven subcategories represented the meaning units of the central message of the rationales for the treatment belief. Participants with positive beliefs (believed entirely/much, n = 244) presented rationales related to: "Experienced positive effects", "Knowledge regarding effect-mechanisms of acupuncture", and "General trustworthiness of acupuncture". Participants with more negative beliefs (believed a little or not, n = 48) presented rationales related to: "Lack of feasibility of the acupuncture", "Varying effects", and "The effect is individual, not available for everybody". Conclusion: In order to strengthen acupuncture treated patients beliefs in the efficacy of acupuncture during clinical practice or research, acupuncture therapists may consider emphasizing these aspects in the therapeutic situation.