Background: Outcomes measured in clinical trials should be meaningful to patients, healthcare professionals and researchers, yet there is heterogeneity in the outcomes used across trials. This inconsistency impacts on the ability to compare findings and may mean that the results have little importance to healthcare professionals and the patients that they care for. The aim of the present study is to review the outcomes used in registered trials of therapies for type 2 diabetes mellitus as the first step in the development of a core outcome set for effectiveness trials in type 2 diabetes. Methods: A systematic review of clinicaltrials.gov entries was completed for randomised, open (actively recruiting or in follow-up period), phase 3 and 4 trials of type 2 diabetes mellitus in adults. Trials of the treatment of diabetes complications, co-morbidities, prevention and surgery were excluded. Each trial was screened for eligibility and outcomes extracted from the primary and secondary outcomes data fields and free text study information. The outcomes were recorded verbatim and classified into core outcome domains according to the COMET taxonomy. Results: A total of 354 trial registrations were reviewed for eligibility and 138 trials included. In total, 1444 outcomes were extracted with a median of eight outcomes per trial (range = 1-60). Outcomes were categorised into 30 different outcome domains according to the COMET taxonomy, but no single domain or outcome was measured in 100% of trials. The majority of trials (88%) included outcomes in the 'metabolism and nutrition' domain, such as lipids and lipoproteins (21%), HbA1c (18%), hypoglycaemia (14%), fasting plasma/blood glucose (11%), glycaemic variability (8%), postprandial response (8%) and insulin sensitivity (5%). Only 10% of trials included one or more patient reported outcomes; of these, 29% included the Diabetes Treatment Satisfaction Questionnaire. Conclusions: There is marked heterogeneity in the outcomes measured in registered therapeutic intervention trials for type 2 diabetes. The use of an agreed set of core outcomes will improve the consistency of reporting in clinical trials for type 2 diabetes. Trial registration: The core outcome set study, of which this is a part, is registered in the COMET database, http://www.comet-initiative.org/studies/details/956. Registered on 24 January 2017.