Summary: The aim of the study was to investigate the effects of booking body mass index (BMI) on obstetric and neonatal outcomes in an inner city UK tertiary referral maternity centre. The Guy's and St Thomas' Maternity and Gynaecology (Terranova Healthware) Database was studied. All women that delivered at St Thomas' during 2005 with normal and high BMI were included in the study. Subjects were divided into three groups: BMI 19 -24.9 (normal); 25-29.9 (overweight) and 30 or greater (obese). Groups were compared using Stata Statistical software. The study included 3642 patients: 2169 normal, 945 overweight and 528 obese. Both overweight and obese groups had a statistically significant association with gestational diabetes (odds ratio [OR] 5.7 and 11.6), hypertension in pregnancy (including preeclampsia [ORs 1.5 and 2.4], preterm rupture of membranes (ORs 3.7 and 5.0) and preterm delivery (ORs 1.4 and 1.6). The rate for caesarean delivery was increased in both overweight and obese women (ORs 1.4 and 1.7). Obesity is an independent risk factor for adverse obstetric outcomes and is significantly associated with caesarean section delivery.