A case of meconium peritonitis that was diagnosed ultrasonographically in the early third trimester in a patient with acute polyhydramnios is presented. The pathophysiologic progression of this condition is demonstrated by serial ultrasound examinations. Neonatal radiologic confirmation of the diagnosis is also presented. The obstetric and neonatal implications of meconium peritonitis and the importance of serial ultrasonography in those pregnancies in which this diagnosis is made are discussed.