A nosocomial diarrhea outbreak caused by Escherichia coli 0153: H45, which produces a thermostable enterotoxin, in five neonates admitted to the neonatology ward of the pediatric service of the Valencia University Hospital is described. The outbreak was discovered during a prospective study lasting eight months and aimed at evaluating the importance of Escherichia coli with enterotoxic capacity in acute infant diarrhea within our environment. The study involved conventional enterotoxigenicity tests applied both in vivo and in vitro. One of the patients, admitted with acute diarrhea was the source of the outbreak, with a possible person to person transmission. The diarrhea was slight to moderate. Emphasis is placed on the importance of this type of diarrhea in developed countries, and the problem is analyzed by reviewing its situation to the present.