Mutations in the human telomerase RNA (TERC) occur in autosomal dominant dyskeratosis congenita (DKC). Because of the possibility that TERC mutations might underlie seemingly acquired forms of bone marrow failure, we examined blood samples from a large number of patients with aplastic anemia (AA), paroxysmal nocturnal hemoglobinuria (PNH), and myelodysplasia (MDS). Only 3 of 210 cases showed heterozygous TERC mutations: both nucleotide 305 (n305) (G>A) and n322 (G>A) were within the conserved region (CR) 4-CR5 domain; n450 (G>A) was localized to the boxH/ACA domain. However, only one patient (with a mutation at n305 [G>A]) had clinical characteristics suggesting DKC; her blood cells contained short telomeres and her sister also suffered from bone marrow failure. Another 21 patients with short telomeres did not show TERC mutations. Our results suggest that cryptic DKC, at least secondary to mutations in the TERC gene, is an improbable diagnosis in patients with otherwise typical AA, PNH, and MDS.