We describe the phenotypes of two male sibs with partial monosomy of chromosome 5 [46,XY,der(5)inv ins(1;5)(p32;q35.4q34)]; maternally derived from a balanced insertion of 1 and 5 [inv ins (1;5)(p.32;q35.4q34)]. One sib had microcephaly, cleft lip and palate, facial anomalies, atrial (ASD) and ventricular (VSD) septal defects, camptodactyly 4th and 5th fingers, and developmental delay. The other sib showed microcephaly, facial anomalies, ASD, hypotonia, primary optic nerve hypoplasia, and developmental delay. Only seven other patients with 5q deletions distal to 5q33 have been reported and none showed the putative breakpoints identified in our two patients. All nine showed developmental delay or malformations of the CNS and facial anomalies; six of nine had defects of cardiac septation. Our two patients and one other were shown to have only one copy of the cardiac specific hCSX gene that defines in part the etiology of their ASD and VSD. The other components of their phenotypes cannot be related at present to genes identified in the deleted segments. © 2001 Wiley-Liss, Inc.