Cross-sectional echocardiographic measurements of normalized septal curvature (NSC), systolic anterior motion of the center of the left ventricular cavity (CAM), and the M mode ratio of left ventricular posterior wall epicardial motion (PEM) to posterior wall thickening (PWT) were made in eight normal subjects, 16 patients with right ventricular volume overload (RVVO) and five with pressure overload (RVPO). Paradoxical M mode septal motion was confined to early systole in six patients with RVVO (group I) and was sustained in 10 (group II). Similar end-diastolic septal flattening was observed in RVVO group I (NSC 0.50 + 0.16 [SDI) and group 11 (0.49 + 0.23) when compared with the normal group (0. 83 + 0.07, both p < .005). NSC increased in both RVVO groups during the first one-third of systole (p < .002) to values not significantly different from normal, but did not change significantly thereafter. CAM in RVVO group 11 (5.4 ± 2.2 mm) exceeded CAM in both the normal group (1.8 + 1.9 mm, p < .001) and group 1 (2.1 ± 1.4 mm, p < .005). Similarly, the PEM/PWT ratios in group LI (mean 2.94; range 2.13 to 8.0) exceeded those in both the normal group (mean 1.59; range 1.11 to 2.13, p < .01) and group I (mean 1.32; range 1.10 to 1.67, p < .01). In the RVPO group, CAM was insignificant, the PEM/PWT ratios were lower than normal (p < .01), and marked enddiastolic septal flattening was incompletely corrected during early systole, after which the septum was flattened further until end-systole (p < .005). Midsystolic septal flattening was also observed in those with RVVO in whom the right ventricular/left ventricular peak systolic pressure ratio exceeded 0.4. Thus, true septal paradox in patients with RVVO is an early systolic event. The apparent persistence of septal paradox beyond early systole is an artifact due to anterior left ventricular translation. These findings help explain the preservation of left ventricular systolic function in RVVO despite apparently sustained septal paradox, support the role of the transseptal pressure gradient in determining septal curvature, and indicate that regional wall motion analysis from a fixed reference point in patients with RVVO is inappropriate.