Fifty-eight patients with ventricular septal defect (VSD) associated with severe pulmonary hypertension (Pp/Ps > or = 0.90) were repaired between 1971 and 1992. Their preoperative Pp/Ps, Rp/Rs and Rp were 0.98 +/- 0.06, 2.37 +/- 1.20 and 4.81 +/- 3.06 units.m2, respectively. Late results were analyzed in 56 operative survivors. The age at the time of operation ranged from 2 months to 32 years (average 4.1 years) and the postoperative follow-up period ranged from 1 month to 20 years (average 5.5 years). Eighty-two percent of the patients were in New York Heart Association functional class I, 15% were in class II and 3% in class III. The postoperative Pp/Ps and Rp/Rs significantly decreased to 0.41 +/- 0.13 (p < 0.001) and 0.25 +/- 0.16 (p < 0.001), respectively. There were significant differences in Rp/Rs and Rp between the patients operated on before (Group 1) and after 2 years of age (Group 2). Rp/Rs and Rp in Group 1 were 0.17 +/- 0.06 and 2.52 +/- 0.65 units.m2, whereas 0.31 +/- 0.19 (p < 0.05) and 4.26 +/- 1.88 units.m2 (p < 0.05) in Group 2, respectively. One patient died 14 months after VSD closure due to respiratory failure. It is concluded that a patient with VSD associated with severe but reversible pulmonary hypertension should be surgically corrected before 2 years of age.