A retrospective analysis of infants with bronchopulmonary dysplasia requiring prolonged hospitalization (greater than 100 days) was carried out to determine those factors associated with fatal outcome. Twenty-three infants made up the study population. Eleven infants died and 12 survived (survivors). No differences were noted between the groups regarding ventilator requirement, radiographic changes, and medication use (digoxin, aldactazide), except for furosemide which was used twice as frequently in the group of infants who died v the group of infants who survived (P less than .001). Differences noted between the groups included moderate hypochloremia (chloride less than 80 mEq/L) in all 11 infants who died v six of 12 survivors, severe hypochloremia (chloride less than 70 mEq/L) in the nine of 11 infants who died v two of 12 survivors, metabolic alkalosis (pH greater than 7.45) in nine of 11 infants who died v three of 12 survivors, hypertension (systolic BP greater than 113 mm Hg) in eight of 11 infants who died v one of 12 survivors, decrease in head growth in ten of the 11 infants who died v one of the 12 survivors; these differences were all significant (P less than .001). The metabolic alkalosis and head growth changes appear to be related to the hypochloremia. The data suggest that chloride deficiency may be an important contributing factor in the genesis of poor outcome in infants with bronchopulmonary dysplasia and that close attention to chloride supplementation might influence outcome.