Vasodilatory shock is common in critically ill patients and vasopressors are a mainstay of therapy. A meta-analysis suggested that use of a higher, as opposed to a lower, mean arterial pressure target to guide titration of vasopressor therapy, could be associated with a higher risk of death in older critically ill patients. The 65 trial is a pragmatic, multi-centre, parallel-group, open-label, randomised clinical trial of permissive hypotension (a mean arterial pressure target of 60 -65 mmHg during vasopressor therapy) versus usual care in critically ill patients aged 65 years or over with vasodilatory hypotension. The trial is conducted in 2600 patients from 65 United Kingdom adult, general critical care units. The primary outcome is all-cause mortality at 90 days. An economic evaluation is embedded. The 65 trial received favourable ethical opinion from the South Central - Oxford C Research Ethics Committee and approval from the Health Research Authority. The results will be presented at national and international conferences and published in peer-reviewed medical journals. Trial registration: ISRCTN10580502