This report presents the results of short- and long-term effects of diazoxide on the pulmonary vascular resistance (PVR) in patients with pulmonary vascular obstructive disease (PVOD) secondary to delayed surgical correction of ventricular septal defect (VSD). Six children, aged 4 to 10 years, who had closure of VSD and who on lung biopsy showed PVOD, received intravenous diazoxide (1, 2, 4, 5, and 6 mg/kg) after measurements of pulmonary artery pressure (PAP), pulmonary flow index (PFI) by thermodilution technique, and PVR were made. The PAP (51 +/- 19 mm Hg versus 45 +/- 16 mm Hg) and PVR (11.5 +/- 2.7 units versus 8.9 +/- 3.8 units) though decreased, did not attain statistical significance (p greater than 0.1). The PFI (3.2 +/- 1.1 versus 3.4 +/- 0.6 L/min/m) did not change (p greater than 0.1). After oral diazoxide (5 to 10 mg/kg per day) for 20 +/- 3.4 months, the PAP (50.8 +/- 19 mm Hg) and PVR (10.6 +/- 5.8 units) did not decrease (p greater than 0.1), but PFI (4.4 +/- 0.9 L/min/m2), though increased, did not attain significance (p greater than 0.05). However, one of these children who had a fall in PVR from 8.5 to 4.5 units following intravenous diazoxide also had a lower resistance (3.9 units) after 20-months of oral diazoxide therapy and improved symptomatically. No complications other than hypertrichosis were encountered on long-term therapy. Based on this experience, it is suggested that oral diazoxide therapy may be used to improve PVOD if there is favorable response to intravenous diazoxide.