Subpartal and neonatal blood gas analyses have attracted increasing interest during the past 20 years. Different studies have been carried out to investigate the causes and immediate consequences of perinatal acidosis. It was the aim of this study to examine the long term outcome of acidotic-born babies. During 16 consecutive months all deliveries in the Obstetric Department of the Centre Hospitalier Universitaire Vaudois (CHUV) were investigated with regard to incidence and causes of a perinatal acidosis (pHa.umb. less than 7.15). The psycho-motor development of all acidotic newborns was followed up for an average of 15 months. Out of 1922 deliveries a blood sample was available in all but 11 cases (0.57%). Seven newborns were excluded from the study on the assumption that their acidosis and outcome might be related to the underlying condition (congenital malformation and infection, extreme prematurity) rather than perinatal events. From the remaining 1904 deliveries 6.4% (N = 121) had a pHa.umb. less than 7.15 (Tab. I). The incidence of certain perinatal factors was compared in the acidotic and the non acidotic groups (Tab. II). The percentage of acidotic newborns is significantly higher in primipareae, in deliveries done on peridural analgesia in cord complications and premature rupture of the membranes, and in forceps deliveries. No difference could be found with regard to multiple pregnancies, meconiumstained amniotic fluid, cesarean sections, prematurity of 28-37 weeks, and sex. The relationship between CTG score and pHa.umb. is summarized in Tab. III.(ABSTRACT TRUNCATED AT 250 WORDS)