A prospective study was undertaken to determine the relationship between previous caesarean section (CS), placenta praevia and placenta praevia accreta. Of 41,206 consecutive deliveries 1851 had had previous caesarean section and 222 had placenta praevia. Of the cases of placenta praevia, 175 occurred in the uterus and 47 occurred after previous CS. Placenta praevia complicated 2.54% of cases with a previous caesarean section compared with 0.44% of cases with no scar--a 5-fold increase. In patients with placenta praevia occurring with a previous scar, 18 were complicated by placenta accreta (38.2%) compared with only 8 (4.5%) in unscarred uteri. After one caesarean section, placenta praevia was complicated by accreta in 10% of cases and after two or more this was 59.2%. The risk of hysterectomy with placenta praevia and uterine scar was 10% but with placenta praevia accreta it was 66%. There was one maternal death in the placenta praevia accreta group.