Echocardiographic characteristics of the interventricular septum (IVS) have been studied in eight patients with chronic constrictive pericarditis (CP). Values of septal thickening (ST) were clearly below normal in all cases. Interventricular septal systolic motion (IVSSM) was normal in four cases, hypokinetic in three and paradoxical in one. In seven out of the eight patients, an early interventricular septal diastolic motion (IVSDM) consisting of a sudden anterior displacement followed by a brisk posterior rebound was recorded. The beginning of this anomalous movement was coincident with the pericardial knock in the phonocardiogram and its peak was coincident with the simultaneously recorded deep "y" trough in the jugular pulse tracing. The tendency toward normality of IVSDM observed after pericardiectomy in six out of seven patients suggests that this peculiar interventricular septal systolic motion may be a frequent and probably specific echocardiographic finding in constrictive pericarditis.