Although fetal heart rate (FHR) monitoring is widely used for fetal surveillance, there is still disagreement about the value of cardiotocography and the interpretation of FHR patterns. We addressed this in a prospective study in which umbilical cord blood gas and pH values at the time of cesarean section were related to different FHR patterns classified prior to surgery. The patient population consisted of 56 pregnant women with singleton fetuses of gestational age between 37 and 43 weeks. Indication for cesarean section was late FHR decelerations in 16 cases and variable decelerations in 17. Twenty-three patients operated on electively, and without any FHR abnormalities, served as a reference group. Late FHR decelerations were associated with the lowest pH and PO2 levels in arterial as well as in venous blood. In the group with variable decelerations, the umbilical vein PO2 was similar to that of the reference group, whereas the PO2 value in the artery was significantly lower. Hence, in this clinical study a clear relationship was found between the different FHR patterns and changes in the acid-base balance.