We assessed the prognostic impact of TET2 mutations and mRNA expression in a prospective cohort of 357 adult AML patients < 60 years of age enrolled in the European Organization For Research and Treatment of Cancer (EORTC)/Gruppo Italiano Malattie Ematologiche dell’ Adulto (GIMEMA) AML-12 06991 clinical trial. In addition the co-occurrence with other genetic defects and the functional consequences of TET2 mutations were investigated. TET2 mutations occurred in 7.6 % of the patients and were an independent marker of poor prognosis (p = 0.024). TET2 and IDH1/2 mutations strongly associated with aberrations in the DNA methyltransferase DNMT3A. Functional studies confirmed previous work that neither nonsense truncations, nor missense TET2 mutations, induced 5-hydroxymethylcytosine formation. In addition, we now show that mutant TET2 forms did not act in a dominant negative manner when co-expressed with the wild-type protein. Finally, as loss-of-function TET2 mutations predicted poor outcome, we questioned whether low TET2 mRNA expression in cases of AML without TET2 mutations would affect overall survival. Notably, also AML patients with low TET2 mRNA expression levels showed inferior overall survival.