Randomized controlled trials (RCTs) are the gold standard for evaluating efficacy of treatments under real world conditions and, as such, it is important that they are conducted with methodological rigour to prevent biased results. Many medical journals have adopted a standard checklist for reporting of RCTs, the CONSORT statement. The objective of this study was to evaluate clinical trials in livestock populations to assess methodological quality and completeness of reporting and to investigate the association between these criteria and treatment effects. A total of 100 clinical trials published between 2006 and 2008 in the English language were randomly selected. For each trial, 2 reviewers independently completed a checklist based on the CONSORT statement and a different 2 reviewers completed a standard template describing the outcomes used and the statistical significance of all reported treatment effects. Disagreements among reviewers were resolved by consensus. The results showed that there were substantive deficiencies in the reporting of many of trial features, both related to methodological quality and completeness of reporting. Details on key features such as randomization, double blinding, and the number of subjects lost to follow-up were reported in only 67, 4, and 62% of trials, respectively. Reporting of random allocation to treatment group was associated with a lower proportion of positive treatments effects within trials, as was reporting of inclusion/exclusion criteria for study subjects, details on the intervention, animal signalment, significance tests of baseline differences for at least one variable, and the methods used to measure all outcomes. The results suggest that there are deficiencies in the current reporting of important features of RCTs conducted in livestock species and that these deficiencies may be associated with biased treatment effects. The creation and adoption of standards for trial reporting in livestock could aid authors, reviewers, and editors in ensuring that necessary trial details are reported in all published trials.