Eight patients with acute polyhydramnios were encountered in 31 103 pregnancies, an incidence of 1 in 3888. All patients with acute polyhydramnios had monozygous twin pregnancies and 14 of the 16 infants were normal. The condition occurred in the second trimester and usually ended in premature delivery within a few days. The perinatal mortality was 100 per cent and accounted for 14.9 per cent of the perinatal mortality in twins.