Pretreatment plasma dilator prostaglandin levels were measured in 16 premature infants with patent ductus arteriosus in an attempt to correlate abnormally elevated levels with clinical responsiveness to indomethacin therapy. Nine of the 16 infants responded well to indomethacin, with complete disappearance of their murmurs by 48 h. Eight of these nine infants had elevated baseline 6 keto PGF1 alpha levels (greater than 500 pg/ml). In contrast, seven of the 16 infants did not respond to indomethacin, and six of these had 6 keto PGF1 alpha within the normal range (less than 500 pg/ml). PGE2 levels varied in the same general direction, but lacked the specificity and sensitivity of the 6 keto PGF1 alpha levels. Thus, 6 keto PGF1 alpha levels seem to correlate with, and may eventually be helpful in predicting, clinical indomethacin responsiveness in the premature neonate with patency of the ductus arteriosus.