Fifteen consecutive cases of polyhydramnios (PH) out of a total number of 8806 deliveries performed between the 28th and the 41st week of pregnancy were investigated during the period from 1979 to 1985. Three cases of acute PH and 12 cases of chronic PH of which 10 were idiopathic were distinguished. From the time the clinical diagnosis was established until delivery, maternal serum and amniotic fluid alpha-fetoprotein (AFP) and prolactin (PRL), as well as maternal serum oestrogens (OT), placental lactogen (HPL), chorionic gonadotropin (hCG) and pregnancy-specific beta 1-glycoprotein (SP1), were studied. A very high increase of maternal serum AFP in all the cases of PH was observed (p less than 0.001) and was associated with the high degree of risk for obstetrical complications, while, on the other hand, amniotic fluid AFP was increased only in PH associated with congenital abnormalities of the fetus. Maternal serum PRL was not different from normal values (p greater than 0.2), while amniotic fluid PRL was lower in all the cases of chronic idiopathic polyhydramnios studied. The protein hormones and the oestrogens showed a discrepancy in their elevation, according to the kind of PH, the outcome of pregnancy, and the condition of the infant at birth.