The records of 33 infants weighing 5 kg or less who received acute hemodialysis treatment at Children's Hospital between 1980 and 1991 were reviewed. Dialysis was initiated to treat hyperammonemia (8), primary renal or renovascular disease (7), and acute renal failure (18). The infants weighed 2.2 to 4.0 kg at birth and 27% were born prematurely. The infants were 2 to 120 days of age (median 10 days) and weighed 2.2 to 5.0 kg (median 3.5 kg) at the initiation of hemodialysis. Hemodialysis access was achieved via double-lumen 7 French catheters in 49% of the infants, the ECMO circuit in 24%, and the umbilical vessels in 27%. Thirty-three infants underwent a total of 216 hemodialysis treatments. Only nine treatments were discontinued prematurely: six for intractable hypotension and three for technical problems. Fifty-two percent (17 of 33) of the infants survived through the end of the hemodialysis treatment course. The survival rates for the infants with hyperammonemia (75%) and primary renal disease (71%) were better than those for infants with acute renal failure (33%). The survivors did not differ from those who died with respect to birthweight, weight when hemodialysis was initiated, or the number of hemodialysis treatments administered. We conclude that infants weighing less than 5 kg can be treated successfully with hemodialysis. Patient survival is related to underlying medical problems, not to complications of hemodialysis.