Surface electrocardiographic (ECG) features of aberrant ventricular conduction (VAb) were correlated with His bundle electrograms (HBE) in 16 patients to assess the usefulness and limitations of ECG in the diagnosis of VAb. VAb was studied in these patients using the technique of atrial premature stimulation (A2) during paced atrial cycle lengths (A1). VAb in the form of right bundle branch block (RBBB) was noted in 12/16 patients, left (L) BBB in 7/16, whereas 3/16 showed R as well as LBBB following A2. The longest atrial coupling (A1A2) intervals resulting in VAb approximated 50% of the basic cycle length (BCL) and VAb was frequently abolished at closer A1A2 intervals due to excessive AV nodal delays. While the PR interval preceding an aberrant QRS complex always exceeded the PR of the prior CL, the two major components of the PR [i.e. AV node and His Purkinje system (HPS)] showed different and unpredictable degrees of delays. In comparison to the beats of BCL, during VAb the AV nodal delays amounted to a maximum of a twofold increase, whereas a greater than fourfold increase was noted in the HPS in some instances. The constant finding of PR prolongation and frequent conduction delay in the HPS (HV prolongation) associated with VAb suggested that the appearance of an unexpectedly shortened PR interval, though not usually considered in the differentiation of VAb from premature ventricular beats, may prove extremely useful; yet the often utilized R-R interval relationship poorly reflects the H-H interval relationship (which determines VAb) and therefore may be frequently unreliable.