Fifty-six patients with "poor R wave progression" (PRWP) on the electrocardiogram (ECG) were studied by Frank vectorcardiography in an attempt to define discriminators for subgroups. Criteria were prospectively applied to a test group with achievement of comparable sensitivity and specificity. Four subgroups were identified in the patients with PRWP: Group I - anterior myocardial infarction (AMI) (20/56, 35%); Group II - left ventricular hypertrophy (LVH) (8/56, 14%); Group III - right ventricular hypertrophy (RVH) (7/56, 13%); AND Group IV - leftward axis (LA) (21/56, 38%). The best discriminator for vectorcardiogram (VCG) AMI was RV3 less than or equal to 1.5mm or R1 less than or equal to 4.0mm (18/20, 90% sensitivity; 26/36, 72% specificity). RVH and LVH were identified by R1 less than or equal to 4.0mm and S1 greater than or equal to 1.0mm and standard LVH voltage criteria respectively. Group IV patients (PRWP without VCG criteria for AMI, LVH or RVH) were identified by exclusion. "Reversed R wave progression" (RRWP) was more specific for AMI than was simple PRWP. The same discriminators, however, were applicable. It is clinically useful to note that 72% (26/36) of patients with either PRWP or RRWP who did not exhibit VCG AMI could be identified by 12 lead ECG with only 2/28 (7%) VCG false negative AMI.