The data available on the prognosis of the intraventricular conduction disturbances--left and right bundle branch block (LBBB, RBBB) and left anterior and posterior hemiblock (LAH, LPH)--are uneven. However, there is no evidence to suggest that incomplete RBBB, established complete RBBB or established LAH carry other than a normal prognosis. Data on incomplete LBBB and on established LPH are insufficient to support conclusions. There are no adequate data on the risks of recently acquired LAH or LPH. Recently acquired isolated RBBB appears to carry little or no additional risk but the data are scant. It requires evaluation and may justify restriction to multi-crew operation. Established LBBB carries a small but definite increase in mortality risk, but subject to assessment it may be permissible in multi-crew operation. Recently acquired LBBB carries a substantial increase in mortality risk and should be disqualifying to fly.