M-mode echocardiograms were obtained in 48 patients with complete left bundle branch block (LBBB). Of these 48 patients, 28 had LBBB with normal frontal plane QRS axis (-20 degrees to +90 degrees, mean +/- SD 18 degrees +/- 34 degrees), and 20 had LBBB with a left axis deviation (LAD) (-30 degrees to -60 degrees, mean +/- SD -48 degrees +/- 11 degrees). In the group with LBBB and normal axis, 25 patients had typical early mean +/- SD -48 degrees +/- 11 degrees). In the group with LBBB and normal axis, 25 patients had typical early systolic posterior septal motion characteristic of LBBB. Septal motion following early posterior septal motion (through the ejection period) was posterior in 24 patients (86%), anterior (paradoxical) in 2 (7%), and flat in 2 (7%). In the group with LBBB and LAD, 16 patients had the typical early systolic posterior septal motion; subsequent septal motion was posterior in 3 (15%), anterior (paradoxical) in 13 (65%), and flat in 4 (20%). Patients with LBBB and normal axis had a higher frequency of posterior septal motion, and patients with LAD had a higher frequency of anterior septal motion (p less than 0.001). The correlation of abnormal axis and paradoxical septal motion may be explained by the activation pattern producing LAD or by a septal disease process producing both abnormalities of axis and abnormal septal motion.