To determine if prolonged active phase of labour is associated with increased risk of uterine scar rupture in labour following previous lower segment caesarean section, a retrospective cohort study (1988-91) was done to analyse active phase partographs of 236 patients undergoing trial of labour following caesarean section, 7 (3%) of whom had scar rupture. After onset of active phase (3 cm cervical dilatation), a 1 cm/h line was used to indicate "alert". A zonal partogram was developed by dividing the active phase partographs into 5 time zones: A (area to the left of "alert" line), B (0-1 h after "alert" line), C (1-2 h after "alert" line), D (2-3 h after "alert" line) and EF (> 3 h after "alert" line). The relative risk of uterine scar rupture was calculated for different partographic time zones. The relative risk of uterine scar rupture was 10.5 (95% confidence interval 1.3-85.5, p = 0.01) at 1 hour after crossing the "alert" line; 8.0 (95% confidence interval 1.6-40.3, p = 0.009) at 2 hours after crossing the "alert" line; and 7.0 (95% confidence interval 1.6-29, p = 0.02) at 3 hours after crossing the "alert" line. In women undergoing trial of labour following caesarean section, prolonged active phase of labour is associated with increased risk of uterine rupture. A zonal partogram may be helpful in assessing this risk in actively labouring women who cross the partographic "alert" line.