To determine the course of right ventricular pressure (RVP) in patients with isolated ventricular septal defect (VSD) and factors influencing it, unselected 148 infants were followed-up longitudinally with color-Doppler echocardiography from a median age of 1 month for 201 patient-years. The patients were divided into three groups by absolute echographic size of VSD: group I, less than or equal to 4.0 mm; group II, >4 to less than or equal to 7 mm; group III, >7 mm. Sixty percent belonged to group I. Muscular defects dominated in group I, perimembranous defects dominated in group II, and those with outlet extensions dominated in group III. Peak systolic RVP was obtained by Doppler-estimated difference between systolic brachial artery and peak gradient across the VSD. Initial RVP ranged between 15 and 95 mmHg and increased in parallel to the size of defect. According to the regression equations RVP decreased in general by 0.17 mmHg per month. This correlated significantly with the size of the defect. In group I, the rate of decrease was Very fast and is best expressed by a log function of time (r = -0.67, r(2) = 0.45). In groups II and III the rate of decrease was less steep and had a greater variability. RVP normalized in 100% in those of group I and in 90% of group II, at median ages of 0.17 and 0.33 years, respectively. Median Q(p):Q(s) values were 1.5, 2.2, and 3.0 in groups I-III, respectively. The outcome depended on the size of VSD. Spontaneous closure was observed in 51% of group I, 10% of group II, and none of group III. The rate was higher in muscular defects. Congestive heart failure was present in 53% and 100% in groups II and Ill, respectively. Death rate was 2.03%, all in patients with large defects. It is concluded that the temporal course of RVP with time can be estimated fairly well by the regression equation presented in relation to the initial size of the VSD.