95 patients were prospectively examined from 1982-1986 in a diagnostic programme. These patients showed a clinically and ultrasonically established acute hydramnios during the second half of their pregnancy period. Possible important causes of an acute hydramnios can be confirmed or excluded via on-target sonography; at the same time, a diabetic condition must be excluded and the amniotic fluid must be examined to exclude acute viral or specific bacterial infections. In 8 cases (8%) we found that the mother had diabetes mellitus requiring insulin, the hydramnios being the main pointer towards the metabolic disturbance. Gestation diabetes was confirmed in 23 patients (25%) on the basis of the polyhydramnios. Chromosomal anomalies were seen in 12 eucyeses (13%) and in 2 polycyeses. In 8 pregnancies (8%) we found a pattern of non-immunological hydrops fetalis with pleural effusions and ascites. In three patients (3%) there was an immunological hydrops fetalis with polyserositis within an Rh incompatibility pattern. Severe foetal malformations associated with a normal karyotype were sonographically found in 12 pregnancies--encyeses and twins--(12%). Cardiac disturbances (tachyarrhythmia) resulted in polyhydramnios in 8 cases of eucyesis (8%). Finally, 10 cases only remained unclarified (11%). 14 cases (15%) had twin pregnancy. In this connection we observed on the average an earlier onset of acute hydramnios (27 weeks) compared with the eucyeses (on the average during the 30th week). Timely and adequate treatment may become possible if we know the aetiology of the acute hydramnios, especially in cases of diabetes mellitus and gestation diabetes, as well as in certain foetal malfunctions, such as e.g. pleural effusions and generalised hydrops fetalis and disturbances of cardiac rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)