The authors report a nosocomial infection outbreak by Klebsiella pneumoniae, observed in neonates at a gyneco-obstetrical hospital from Mexico City. Forty six newborns presented one or more infections due to K. pneumoniae during their stay in neonatal care units, between October 3 and November 12, 1988. Sepsis was documented in 41 cases by clinical picture and routine laboratory exams, including one positive, blood culture at least. The most frequent invasive procedures practiced in these patients were catheterization and ventilatory support. K. pneumoniae was isolated as well from several environmental sources that could have led to infection of patients. Treatment of cases was initiated with ampicillin-amikacin, however, therapeutic failure with a lethality rate of 50% (14/28) and results of antimicrobial susceptibility conducted to treatment with cefotaxime. Fifteen out of 19 patients receiving the cephalosporin survived. To prevent outbreaks like the one presented here, we concluded that appropriate measures dealing with hygiene and education of personnel plus monitoring of bacterial susceptibility to antimicrobials, should prove successful in our environment.