Two cases of subclinical and undiagnosed thyrotoxicosis that presented with unexplained tachycardia during surgery are described. Differential diagnosis and logistics in the management of patients presenting with tachycardia, with or without fever during anaesthesia are discussed. It is emphasised that when encountering unexplained tachycardia during anaesthesia, thyrotoxicosis must be suspected. Investigations for thyrotoxicosis must be carried out in the postoperative period.