Episiotomy is a widely-done intervention in childbirth, regardless of poor scientific evidence of its benefits. This randomised controlled trial compares selective with routine use of a mediolateral episiotomy for women having first and second deliveries in 8 public maternity units in Argentina. 2606 women participated; 1555 were nulliparous (778 in the selective group and 777 in the routine group) and 1051 primiparous (520 in the selective group and 531 in the routine group). The two interventions compared were selective (limited to specified maternal or fetal indications), and routine episiotomy (following the hospital's previous policy). Episiotomy was done in 30.1% of deliveries in the selective, and 82.6% in the routine group. The main outcome measure was severe perineal trauma. Severe perineal trauma was uncommon in both groups but was slightly less frequent in the selective group (1.2% vs 1.5%). Anterior perineal trauma was more common in the selective group but posterior perineal surgical repair, perineal pain, healing complications, and dehiscence were all less frequent in the selective group. Routine episiotomy should be abandoned and episiotomy rates above 30% cannot be justified.