Aim: To assess trends in the epidemiology of culture-proven and clinical neonatal sepsis of vertical transmission in the era of intrapartum antibiotic prophylaxis. Methods: Since 1995, the neonatal services of 28 acute-care teaching hospitals in Spain (“ Grupo de Hospitales Castrillo” ) have been involved in the prospective surveillance of neonatal infection of vertical transmission. We here report the comparison of the incidence for the periods 1996-1997 and 2000-2001, and look separately at two groups of hospitals according to the time at which the official hospital policy to provide intrapartum antibiotic prophylaxis for group B Streptococcus (GBS) prevention was adopted. In 16 hospitals the policy was adopted in 1999 and in 10 before 1998 (nine hospitals in 1996, one in 1997). Results: The incidence of proven sepsis decreased significantly by 22% and 54% in the hospitals that started prophylaxis in 1999 and before 1998, respectively. The incidence of GBS sepsis also declined significantly by 36.4% and 65.4% in both groups of hospitals, respectively. Significant variations in the incidence of clinical vertical sepsis as well as in the incidence of sepsis caused by Haemophilus influenzas and Klebsiella were not found. Sepsis caused by Escherichia coli increased in the hospitals with prophylaxis from 1999 and decreased in those that began prophylaxis before 1998, in which the mortality rate of proven and clinical sepsis also decreased significantly by 58.6% . Conclusions: There was a substantial decline in the incidence of proven vertical sepsis, with a significant reduction of GBS sepsis, although decreases were more marked when antibiotic prophylaxis was started before 1998. In this group of hospitals, there was also a decrease in the mortality rate. Fluctuations in the incidence of E. coli infection suggest the need for continuing epidemiological surveillance. Aim: To assess trends in the epidemiology of culture-proven and clinical neonatal sepsis of vertical transmission in the era of intrapartum antibiotic prophylaxis. Methods: Since 1995, the neonatal services of 28 acute-care teaching hospitals in Spain (“Grupo de Hospitales Castrillo ”) Have been involved in the prospective surveillance of neonatal infection of vertical transmission. We here report the comparison of the incidence for the periods 1996-1997 and 2000-2001, and look separately at two groups of hospitals according to the time at which the official hospital policy to provide intrapartum antibiotic prophylaxis for group B Streptococcus (GBS) prevention was adopted. In 16 hospitals the policy was adopted in 1999 and in 10 before 1998 (nine hospitals in 1996, one in 1997). Results: The incidence of proven sepsis decreased significantly by 22% and 54% in the hospitals that started prophylaxis in 1999 and before 1998, respectively. The incidence of GBS sepsis also declined si Significant variations in the incidence of clinical vertical sepsis as well as in the incidence of sepsis caused by Haemophilus influenzas and Klebsiella were not found. Sepsis caused by Escherichia coli increased in the hospitals with prophylaxis from 1999 and decreased in those that began prophylaxis before 1998, in which the mortality rate of proven and clinical sepsis also significantly significantly decreased by 58.6%. Conclusions: There was a substantial decline in the incidence of proven vertical sepsis, with a significant reduction of GBS sepsis, rather decreased the more marked when antibiotic prophylaxis was started before 1998. In this group of hospitals, there was also a decrease in the mortality rate. Fluctuations in the incidence of E. coli infection suggest the need for continuing epidemiological surveillance .