The influence of pelvic outlet capacity on fetal head presentation in 1402 term primiparas with normal pregnancies was studied. In all cases radiological pelvimetry was carried out and labor started spontaneously. Occiput posterior (OP) delivery occurred in 5.1%. As pelvic outlet capacity decreased an increased frequency of OP presentations and need for epidural anesthesia (EDA) was found. With OP presentation the duration of labour was longer, the frequency of EDA, instrumental delivery, cesarean section and low Apgar score at 1 minute were all higher, all compared with occiput anterior (OA) presentation. No difference in fetal morbidity was found. When the influence of the pelvic outlet capacity was eliminated through comparison of matched groups, the course of delivery became more similar whether the presentation was OA or OP and the frequency of EDA became the same. Reduced pelvic outlet capacity seemed to be one cause of both OP presentation and the use of EDA.