BackgroundOpen access (OA) journals are becoming a publication standard for health research, but it is not clear how they differ from traditional subscription journals in the quality of research reporting. We assessed the completeness of results reporting in abstracts of randomized controlled trials (RCTs) published in these journals.MethodsWe used the Consolidated Standards of Reporting Trials Checklist for Abstracts (CONSORT-A) to assess the completeness of reporting in abstracts of parallel-design RCTs published in subscription journals (n = 149; New England Journal of Medicine, Journal of the American Medical Association, Annals of Internal Medicine, and Lancet) and OA journals (n = 119; BioMedCentral series, PLoS journals) in 2016 and 2017.ResultsAbstracts in subscription journals completely reported 79% (95% confidence interval [CI], 77–81%) of 16 CONSORT-A items, compared with 65% (95% CI, 63–67%) of these items in abstracts from OA journals (P < 0.001, chi-square test). The median number of completely reported CONSORT-A items was 13 (95% CI, 12–13) in subscription journal articles and 11 (95% CI, 10–11) in OA journal articles. Subscription journal articles had significantly more complete reporting than OA journal articles for nine CONSORT-A items and did not differ in reporting for items trial design, outcome, randomization, blinding (masking), recruitment, and conclusions. OA journals were better than subscription journals in reporting randomized study design in the title.ConclusionAbstracts of randomized controlled trials published in subscription medical journals have greater completeness of reporting than abstracts published in OA journals. OA journals should take appropriate measures to ensure that published articles contain adequate detail to facilitate understanding and quality appraisal of research reports about RCTs.