A rare case of retroperitoneal gastric duplication is reported and discussed. An intra-abdominal cyst was detected at 31 weeks gestation and was followed up prenatally as a left sided duplex kidney. Post-natal ultrasound however, showed a normal kidney, but a cyst with features of enteric duplication in the left upper quadrant adjacent and compressing the kidney. Surgery was carried out during infancy and a retroperitoneal cyst was excised that contained heterotrophic gastric mucosa.