We report a case of concealed Wolff-Parkinson-White syndrome detected for the first time during spinal anaesthesia in an adult male. Episodes of tachy'arrhythmia with a heart rate of approximately 115 beat.min−1, wide QRS complexes and negative T waves which lasted 30–60 s, but were unassociated with hypotension, occurred three times after spinal anaesthesia. Postoperative Holler ECG monitoring showed the frequent occurrence of supraventricular premature contractions and paroxysmal supraventricular tachycardias with the same electrophysiological characteristics as those noted during spinal anaesthesia. The patient was diagnosed as having concealed Wolff-Parkinson-White syndrome. Since this condition is asymptomatic and undetectable by routine pre-operative screening it is likely that other unsuspected cases will arise. When episodes of tachyarrhythmia occur unexpectedly during anaesthesia, as was the case in this patient, postoperative examination including Holler ECG monitoring will be necessary to determine the nature and severity of the tachyarrhythmia.