We investigated infants with respiratory distress within 4 days of birth whose mothers had not received antibiotic prophylaxis to evaluate the frequency and etiology of bacterial infection and associated risk factors. The study was conducted on 261 infants suffering respiratory distress admitted to a Brazilian neonatal intensive care unit, 94 per cent of whom were born prematurely. Gestational and delivery history; bacteriological cultures of blood, cerebrospinal fluid, tracheal aspirates and urine; complete and differential blood counts; a urinary group B streptococcal latex antigen test; and a chest radiograph were analysed. Indications of infection were found in 38.7 per cent and confirmed in 11.9 per cent of the neonates. Gram-positive (70.9 per cent) and gram-negative bacteria (29.1 per cent) were found in 31 cases of confirmed early bacteremia. Group B Streptococcus was the predominant causative agent (19.4 per cent) in infants exhibiting confirmed infection. Culture-proven infection was more frequent among infants delivered vaginally (adjusted OR = 2.53, p = 0.05) or born to mothers with signs of intra-amniotic infection (adjusted OR = 2.83, p = 0.04). Preventive measures against early bacterial infection in preterm infants from this population are strongly warranted.