ICF syndrome has been described as the association of variable immunodeficiency, facial anomalies and centromeric heterochromatin instability. Since the chromosome rearrangements seen in cells of ICF patients are reminiscent of the chromosomal changes induced by the undermethylating agent 5-azacytidine in the late S-phase, we have analyzed the methylation pattern of satellite sequences in four patients. These sequences are almost completely methylated in normal leukocyte DNA. When ICF DNA was tested with methyl-sensitive enzymes, several classical satellite families, but not alphoid sequences, showed a very low level of methylcytosine in leukocyte DNA, with an abnormal pattern compared to the normal germinal and extraembryonic methylation profile. The methylation deficiency affects classical satellite families built from distinct unit sequences but located in the same chromosomal region. This observation may have important implications for the mechanism of chromosomal rearrangements.