The idiopathic Tn-syndrome, formerly called ‘permanent mixed-field polyagglutinability’, is a rare hematological disorder characterized by the expression of the Tn-antigen on all blood cell lineages. The immunodominant epitope of the Tn-antigen is terminal α-N-acetylgalactosamine, O-glycosidically linked to protein. Normally this residue is 3′-substituted by β-galactose thereby forming the core 1 structure known as the Thomsen–Friedenreich (TF) antigen (Galβ1⇒3GalNAcα1⇒Thr/Ser). The cause of the exposure of the Tn-antigen appears to be due to the silencing of the gene expression of β1,3galactosyltransferase, since treatment of deficient Tn(+) lymphocyte T clones with 5′azacytidine or Na butyrate leads to reexpression of enzyme activity and the sialylated TF-antigen. The Tn-syndrome is acquired and permanent and affects both sexes at any age. Its origin is unknown. Pluripotent stem cells are affected since all lineages are involved but each one to a variable extent. Therefore, normal cells co-exist with Tn-transformed cells. Clinically, patients suffering from the Tn-syndrome appear healthy. Laboratory findings usually reveal moderate thrombocyto- and leukopenia and some signs of hemolytic anemia not warranting any treatment.