Prolonged low dose indomethacin for persistent ductus arteriosus of prematurity. A total of 121 infants who required indomethacin for persistent ductus arteriosus in Liverpool and Cambridge over a four year period were randomised to receive either 0.1 mg/kg daily for six days or 0.2 mg/kg every 12 hours for three doses. The groups were of similar birth weight and gestational and postnatal age, though those treated with a low dose were by chance receiving a higher percentage of oxygen at the start of treatment and there were more deaths from bronchopulmonary dysplasia in this group. Of 59 infants treated with the prolonged course 53 (90%) responded initially to indomethacin compared with 48 of 62 (77%) treated conventionally--a difference of 13% (95% confidence interval for the difference 0 to 26%). Of the 53 responders 11 (21%) relapsed after low dose indomethacin, whereas after the shorter course 19 of 48 (40%) relapsed. This difference was significant (95% confidence intervals 3 to 37%). Side effects, mainly gastrointestinal haemorrhage, were similar in both groups. Significantly fewer infants experienced a rise in serum creatinine or urea concentration after treatment with low dose indomethacin. A prolonged low dose course of indomethacin offers advantages over conventional treatment. 