The frequency and likely aetiology of systemic arterial hypotension occurring either during or following operation, in patients receiving steroids was analysed retrospectively. Hypotension occurred during surgery in 29 of 250 operations. Although patients receiving steroids until shortly before operation appeared more likely to develop hypotension during operation the differences were eliminated when allowance was made for the severity of the disease. In almost half of the instances of hypotension during surgery there was no adequate explanation. Hypotension occurred after surgery on eight occasions; there was one case of acute adrenocortical insufficiency, six of various types of circulatory failure and in one the cause as uncertain.