Objectives: To describe and quantify the maternal and neonatal factors associated with Group B streptococcus (GBS) disease in infants <90 days of age. Setting: Neonatal Units in London, Oxford, Portsmouth and Bristol. Patients: Cases were infants <90 days of age with invasive GBS disease diagnosed between 2000 and 2003, and controls were healthy infants born in the same hospital and in the same birth weight category. Main outcome measures: Demographic and clinical data on the mother, baby, birth and neonatal stay. Results: 138 cases and 305 controls were recruited. The majority of cases (74%) presented in the first week of life (early onset, EO); most on day 1 (89%). 65% of EO cases had one or more clinical risk factors (prematurity, prolonged rupture of membranes (PROM), known maternal GBS carriage, fever during labour). A multivariable logistic regression analysis found that the strongest independent associations with GBS disease were known maternal carriage of GBS (odds ratio (OR) 6.9), maternal infection in the peripartum period (OR 4.2) and maximum temperature in labour (OR 2.2 per °C). GBS disease was associated with twice the likelihood of PROM and fetal tachycardia (p=0.05 and 0.07 respectively). EO cases had lower Apgar scores and were more likely to have respiratory distress and convulsions, and to require tube feeding than controls. They spent longer in hospital than controls, requiring longer stays at all levels of care. Conclusions: Independent of birth weight, a number of maternal, birth and neonatal factors are significantly associated with GBS disease. The management of babies with GBS disease results in an appreciable use of hospital resources.