Literature reports only eleven cases of subclavian steal syndrome after CABPG through a left internal mammary artery (LIMA). In 2 asymptomatic patients the diagnosis was established at routine control angiography. In all cases some degree of stenosis of the subclavian artery (SCA) proximally to the emergence of the LIMA was demonstrated and quickly corrected by means of a left carotido-subclavian bypass graft. We report the observation of a patient who presented already at the 11th postoperative day recurrent instable angina with typical ECG changes. This case is absolutely exceptional for two reasons: Postoperative angina appeared exclusively during sleep but never under exercise. Careful interrogation revealed that the patient always slept with his hands crossed under the neck, in a position comparable to the Adson test. In this critical position of the arms, typical attacks of angina and ECG changes could be easily and repetitively induced. A steal syndrome though the LIMA bypass could be demonstrated at coronary angiography and simultaneous measures of the systolic blood pressure on both arms revealed a difference over 20 Torrs which became apparent exclusively in upright position of the arms. All the symptoms disappeared spontaneously within a few months. Six months postoperatively the ECG was normal even in the upright position of the arms and no blood pressure difference could be evidenced anymore. A control coronary angiography showed a patent and well-functioning LIMA graft on the LAD and the absence of any retrograde perfusion of the LIMA graft. These different transitory symptoms have probably to be related to a postoperative compression of the proximal SCA by hematoma or by oedematous reactions.(ABSTRACT TRUNCATED AT 250 WORDS)