Objective To observe the effect of a trial of instrumental delivery in theatre on outcome for mother and baby. Design A prospective observational study. Methods Relevant maternal and neonatal data were collected for all instrumental deliveries of singleton viable pregnancies delivered over a three month period. Population Two hundred and twenty nine consecutive deliveries conducted by ventouse or forceps because of fetal distress or dystocia. Setting The maternity unit of a teaching hospital delivering around 6000 women annually. Main outcome measures The decision-to-delivery intervals (DDI), mode of delivery and neonatal condition at birth. Results Sixty (26%) deliveries were managed as a trial in theatre, 46 (77%) because of prolonged second stage, with malposition being a factor in 39, and 14 (23%) because of fetal distress. The mean ± SD DDI for these 60 deliveries was 59.2 ± 20.4 minutes (median 58 minutes) compared with 21.2 ± 9.0 minutes (median 20 minutes) for 169 delivered in the labour room (P < 0.0001). Of these 169 deliveries, 168 were delivered within 46 minutes and 1 delivered by caesarean section at 60 minutes. Nine women (13%) ultimately delivered by caesarean section, eight following a trial in theatre; in seven, there was malposition. Deliveries following a trial had slightly less favourable cord blood gas results. Conclusions Trial of instrumental delivery takes two to three times longer than delivery in the labour room; fetal malposition was the major indication for the trial of instrumental delivery and reason for failed delivery. Adopting the recent guidelines of the Royal College of Obstetricians and Gynaecologists, at least 107 (47%) should have been managed as a trial in theatre. The added delay in delivery could be damaging to an already hypoxic fetus, and the use of a trial should be individually assessed.