BackgroundSpin represents specific reporting strategies, either intentional or unintentional, to convince the reader that the beneficial effect of the experimental intervention in terms of efficacy and safety is greater than that shown by the results. The objectives of this study were to 1) develop a classification of spin specific to non-randomized studies assessing an intervention and 2) estimate the prevalence of spin in abstracts of reports of such studies.MethodsIn a first step, we developed a specific classification of spin for non-randomized studies by a literature review and pilot study. In a second step, 2 researchers trained in the field of methodology evaluated the prevalence of spin in the abstract of all non-randomized studies assessing an intervention published in the BioMed Central Medical Series journals between January 1, 2011 and December 31, 2013. All disagreements were resolved by consensus. We also determined whether the level of spin in abstract conclusions was high (spin reported without uncertainty or recommendations for further trials), moderate (spin reported with some uncertainty or recommendations for further trials) or low (spin reported with uncertainty and recommendations for further trials).ResultsAmong the 128 assessed articles assessed, 107 (84 %) had at least one example of spin in their abstract. The most prevalent strategy of spin was the use of causal language, identified in 68 (53 %) abstracts. Other frequent strategies were linguistic spin, inadequate implications for clinical practice, and lack of focus on harm, identified in 33 (26 %), 25 (20 %), and 34 (27 %) abstracts respectively. Abstract conclusions of 61 (48 %) articles featured a high level of spin.ConclusionAbstract of reports of non-randomized studies assessing an intervention frequently includes spin. Efforts to reduce the prevalence of spin in abstract for such studies are needed.Electronic supplementary materialThe online version of this article (doi:10.1186/s12874-015-0079-x) contains supplementary material, which is available to authorized users.