Analysis of clinical and cytogenetic findings taken from 62 published cases of partial trisomies of chromosome 13 showed that 15 had partial trisomy for the proximal long arm and 47 had trisomy for the distal long arm. Persistence of fetal haemoglobin (Hb F), increased projections of polymorphonuclear leucocytes (PMN), depressed nasal bridge, cleft lip/palate, and clinodactyly were more frequent in patients with proximal trisomy 13. In the distal trisomy group, the common features included haemangioma, bushy eyebrows, long curled eyelashes, prominent nasal bridge, long philtrum, thin upper lip, highly arched palate, and hexadactyly. In addition, several other features were common to both the groups, often showing inconsistency even when the same segment was in trisomy. The influence of the second aneusomy as the most likely cause for such inconsistent and overlapping phenotypes is discussed in view of the fact that 42 of 62 cases were derived from a balanced translocation carrier parent.