The occurrence of biliary calculosis as a complication of the use of ceftriaxone was first described in an 18-year-old patient with chronic granulomatosis. Since then many reports have been published on this type of complication both in children and in adults, but until the present moment, this complication had never been reported in pre-term neonates. The authors describe two cases of biliary calculosis associated with the use of ceftriaxone in preterm-newborns, emphasizing that due to the frequent use of this type of antibiotic in neonatal I.C.U., routine ultrosonografic control exams should be performed to diagnose this possible complication in all neonates receiving ceftriaxone.