PURPOSE: Patent ductus arteriosus (PDA) is a common disease in very low birth weight infants (VLBWI). Hemodynamically significant PDA increases the morbidity and mortality of premature infants. Based on experimental model, light inhibits the constriction of immature piglet's ductal rings. No specific mechanism adequately explains the effect of light on the relaxation of PDA. Several hypotheses, including activation of photosensitive metabolites, alterations in receptors, or alterations in prostaglandin metabolism, have been postulated. The purpose of this study was to evaluate the influence of phototherapy on incidence of PDA in VLBWI. Mehtods : Sixty-three infants with birth weights less than 1,500 gm from March 1994 to February 1996 who were admitted in NICU of Seoul National University Children's Hospital were included. Thirty-four infants from March 1995 to February 1996 were shielded with aluminium foils on left chest during phototherapy (Shield group) and twenty-nine infants from March 1994 to February 1995 were not shielded (No shield group : control group). We investigated the incidence and the perinatal risk factors of PDA. RESULTS: 1) The incidence of PDA was 18% in shield group and 41% in control group. There was statistically significant between the two groups (P<0.05). 2) There was not statistically significant between two groups with gestational age, birth weight, sex, delivery mode, etc. 3) The perinatal risk factors which were statistically significant were group and presence of respiratory distress syndrome (RDS), and use of artificial surfactant. With linear logistic regression analysis, only group (OR=8.3, 95% CI=1.17-58.69) and presence of RDS (OR=21.3, 95% CI=1.39-329.81) were proved to be related to the occurrence of PDA. CONCLUSIONS: We conclude that chest shielding during phototherapy is a simple and inexpensive method to decrease the incidence of PDA.