During obstetric ultrasound examinations of 10, 214 patients at a tertiary perinatal unit, 102 cases of mild to severe polyhydramnios were detected, a prevalence rate of 1.04%. When referred patients were excluded, a prevalence rate of 0.93% was obtained. An etiology for polyhydramnios was apparent in 16.5% of patients with mild polyhydramnios. When polyhydramnios was characterized as moderate or severe, a definable cause was determined in 21 of 23 patients (91.3%).