Renal immaturity is pronounced in very low-birth-weight infants with a gestational age < or = 30 weeks. We attempted to elucidate if conditions requiring mechanical ventilation, including patent ductus arteriosus, might further compromise renal function due to decreased renal perfusion. Forty infants studied between 4 and 28 days of age were divided into four groups: Control with no patent ductus or mechanical ventilation (n = 8); PDA+MV, with both patent ductus and mechanical ventilation (n = 17); PDA, with patent ductus (n = 6); MV, with mechanical ventilation (n = 9). The groups PDA+MV and MV had significantly lower creatinine clearances and significantly higher fractional sodium excretions than controls. Mean arterial pressure was significantly lower in all groups compared to controls and correlated significantly with creatinine clearance (r = 0.47, p < 0.02). In conclusion, low renal function in these infants is further compromised by a patent ductus arteriosus and/or the use of mechanical ventilation.