Cytogenetic analysis was carried out in a prospective series of 36 children with DiGeorge syndrome. High-resolution banding (> 850 bands/haploid set) was achieved in 30 cases. Monosomy 22q11.21-->q11.23 was found in 9 of these 30 cases. In each of these cases monosomy 22q11.21-->q11.23 resulted from an interstitial deletion and not from a translocation. No other chromosome abnormalities were seen.