The sex ratios of offspring associated with various types of placental pathology were studied in order to investigate whether this may throw light on the causes of the pathology. The literature was searched for data on the sex ratios of offspring associated with: abruptio placenta, placenta praevia, placenta accreta, extrauterine pregnancy, acute fatty liver of pregnancy or toxaemia of pregnancy. Highly significant male excesses associated with abruption placenta, placenta praevia, fatty liver of pregnancy and toxaemia were noted. Highly significant female excesses were found to be associated with placenta accreta and extrauterine pregnancy. Each type of pathology is associated with a typically biased offspring sex ratio. No explanation of this has ever been offered, so it seems worth postulating that the abnormal sex ratios are caused by abnormal hormone concentrations at conception, and that these are associated with abnormal hormone concentrations later on which are partially responsible for the pathology. The point could be tested by examining the extent to which hormone concentrations control first, Fallopian tube motility, and second, infiltration by extravillous trophoblasts into the placental bed.