Ultrasonography at 23 weeks of gestation documented the presence of megacystis with horseshoe kidney, microcolon, intestinal malrotation, and decreased amniotic fluid volume. After pregnancy termination, an autopsy was performed. The external phenotype was diagnostic of the trisomy 18 syndrome confirmed by chromosome examination. The fetus also had a massively distended bladder with parchment-thin wall, microcolon, intestinal malrotation but no urethral obstruction or hydronephrosis. No ganglion cells were present in the colon or bladder. This has not been mentioned in other reported cases and, therefore, suggests pathogenic heterogeneity. The megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS) is a rare autosomal recessive condition of unknown pathogenesis whose genes map to 15q24. Thus, its previously undescribed presence in trisomy 18 further suggests etiologic heterogeneity.