Anatomical and histopathological findings in 12 cases of trisomy 13 syndrome (nine with classic full trisomy and three with trisomy 13 and an unbalanced Robertsonian 13/13 translocation) are reported. Emphasis is on the brain defects, cardiovascular anomalies, and histological organ dysplasia. Eight patients showed abnormal development of the forebrain and midline facial structures (holoprosencephaly). Cardiovascular malformations were invariably present, the leading malformation being an infundibular ventricular septal defect often in combination with dextroposition of the aorta and abnormalities of the semilunar valves. Histological abnormalities giving evidence of organ dysplasia were observed in the central nervous system, eyes, pancreas, kidneys, and ovaries. Mild cystic renal dysplasia was a constant feature. Foci of persistent nodular renal blastema were found in six cases. The pancreatic dysplasia appears to be pathognomonic for trisomy 13. These observations illustrate the importance of pathological studies in the recognition of chromosome abnormalities and, more specifically, of trisomy 13 syndrome. Based on autopsy data, trisomy 13 can be diagnosed - or ruled out - with certainty, even in the absence of karyotyping.