Little informaition is available concerning the relation between the echocardiographic findings and the pulmonic to systemic flow ratio. Accordingly the author assessed the pattern of the systolic interventricular septal motion and the relation between the type of the systolic interventricular septal motion and the pulmonic to systemic flow ratio in 30 cases with catheterization evidence of secundum atrial septal defect employing the M-mode echocardiographic technique. The results were as follows: 1) Analysis of the pattern of the systolic interventricular septal motion (SESM) allowed classification of cases into 5 groups: Type 1A (4 cases)-Both the right (RS) and left (LS) sides of the interventricular septum move anteriorly during ventricular systole: Type1V (5 cases)-Both RS and LS of the interventricular septum move posteriorly during ventricular systole:Type 2A (9 cases)-LS of the interventricular septum moves posteriorly, while RS fo the interventricular septum remains relatively flat during ventricular systole; Type 2B (8 cases)-RS of the interventricular septum moves anteriorly, while LS of the interventricular septum remains relatively flat during ventricular systole;Type 3(4 cases)-SISM is the same as type 2 and the motion of the left ventricular posterior wall is relatively hyperactive. 2) The pulmonec to systemic flow ratio (Qp/Qs) was statistically different between type 1, 2, and 3 of SISm (Qp/Qs in type 3=2.3+/-0.33;Qp/Qs in type 2=3.4+/-0.79;Qp/Qs in type 3=5.1+/-1.37). 3) Qp/Qs was not different between type 1A and 1B (Qp/Qs in type 1A=2.3+/-0.28;Qp/Qs in type 1B=2.3+/-0.39) and between type 2A and 2B (Qr/Qs in type 2A=2.20+/-0.80; Qp/Qs in type 2B=3.7+/-0.76). 4) Tyep 2 in SISm predicts Qp/Qs> or =2.5(sensitivity=87.5%;specificity=100%;accuracy=90%), and type 3 in SiSm predicts Qp/Qs> or =4.0(sensitivity=40;specificity=100%;accuracy=80%). In conclusion, the pattern of the systolic interventricular septal motion (SISM) in the M-mode echocardiogram may be useful for the semiquantitative assessment of the pulmonic to systemic flow ratio (Qp/Qs) in cases with secundum atrial septal defect.