Using genotype analysis and multiplex quantitative polymerase chain reaction (PCR), chromosome 22q11 deletions were examined in 252 patients with syndromic or isolated conotruncal heart defect. Of these patients, 19 (7.5%) were found to be hemizygous for chromosome 22q11. Parental origin of the deleted chromosome was determined in 16 cases: one patient (6.3%) inherited a deleted chromosome 22 from his mother; all the others (93.7%) consisted of de novo mutations. One-third (5/15) of the de novo 22q11 deletions were of paternal origin and the remainder derived maternally. These results lend further support to our current knowledge of chromosome 22q11 microdeletion syndromes and their implications for the genetic counseling of individuals diagnosed with conotruncal heart defects. Possible mechanisms for gender-biased parental origin are discussed.