BACKGROUND: Eyes sustaining open globe trauma are at high risk of severe visual impairment. Proliferative vitreoretinopathy is the most common cause of retinal detachment and visual loss in eyes with open globe trauma. There is evidence from experimental studies and pilot clinical trials that the use of adjunctive steroid medication triamcinolone acetonide can reduce the incidence of proliferative vitreoretinopathy and improve outcomes of surgery for open globe trauma. OBJECTIVE: The Adjunctive Steroid Combination in Ocular Trauma or ASCOT study aimed to investigate the clinical effectiveness of adjunctive triamcinolone acetonide given at the time of vitreoretinal surgery for open globe trauma. DESIGN: A phase 3 multicentre double-masked randomised controlled trial randomising patients undergoing vitrectomy following open globe trauma to either adjunctive triamcinolone acetonide or standard care. SETTING: Hospital vitreoretinal surgical services dealing with open globe trauma. PARTICIPANTS: Patients undergoing vitrectomy surgery who had sustained open globe trauma. INTERVENTIONS: Triamcinolone acetonide 4 mg/0.1 ml into the vitreous cavity and 40 mg/1 ml sub-Tenon's or standard vitreoretinal surgery and postoperative care. MAIN OUTCOME MEASURES: The primary outcome was the proportion of patients with at least 10 letters of improvement in corrected visual acuity at six months. Secondary outcomes included retinal detachment secondary to proliferative vitreoretinopathy, retinal reattachment, macula reattachment, tractional retinal detachment, number of operations, hypotony, elevated intraocular pressure and quality of life. Health-related quality of life was assessed using the EuroQol Five Domain and Visual Function Questionnaire 25 questionnaires. RESULTS: A total of 280 patients were randomised; 129 were analysed from the control group and 130 from the treatment group. The treatment group appeared, by chance, to have more severe pathology on presentation. The primary outcome (improvement in visual acuity) and principal secondary outcome (change in visual acuity) did not demonstrate any treatment benefit for triamcinolone acetonide. The proportion of patients with improvement in visual acuity was 47% for triamcinolone acetonide and 43% for standard care (odds ratio 1.03, 95% confidence interval 0.61 to 1.75, p = 0.908); the baseline adjusted mean difference in the six-month change in visual acuity was -2.65 (95% confidence interval -9.22 to 3.92, p = 0.430) for triamcinolone acetonide relative to control. Similarly, the secondary outcome measures failed to show any treatment benefit. For two of the secondary outcome measures, stable complete retinal reattachment and stable macular retinal reattachment, outcomes for the treatment group were significantly worse for triamcinolone acetonide at the 5% level (respectively, odds ratio 0.59, 95% confidence interval 0.36 to 0.99, p = 0.044 and odds ratio 0.59, 95% confidence interval 0.35 to 0.98, p = 0.041) compared with control in favour of control. The cost of the intervention was £132 per patient. Health economics outcome measures (Early Treatment Diabetic Retinopathy Study, Visual Function Questionnaire 25 and EuroQol Five Dimensions) did not demonstrate any significant difference in quality-adjusted life-years. CONCLUSIONS: The use of combined intraocular and sub-Tenon's capsule triamcinolone acetonide is not recommended as an adjunct to vitrectomy surgery for intraocular trauma. Secondary outcome measures are suggestive of a negative effect of the adjunct, although the treatment group appeared to have more severe pathology on presentation. FUTURE WORK: The use of alternative adjunctive medications in cases undergoing surgery for open globe trauma should be investigated. Refinement of clinical grading and case selection will enable better trail design for future studies. TRIAL REGISTRATION: This trial is registered as ISRCTN 30012492, EudraCT number 2014-002193-37, REC 14/LNO/1428, IRAS 156358, Local R&D registration CHAD 1031. FUNDING: This project was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (12/35/64) and will be published in full in Health Technology Assessment; Vol. 27, No. 12. See the NIHR Journals Library website for further project information.