Background: Survivors of childhood cancer are at risk for therapy-related subsequent malignant neoplasms (SMN), including thyroid SMN. Telomere length (TL) is associated with cancer risk, but the relationship between TL and SMN risk among survivors is less clear. Methods:Weconducted a nested, matched case-control study of radiation-exposed 15-year adult survivors of childhood cancer with thyroid SMN (cases) and without SMN (controls). Forty-six cases were matched to 46 controls by primary diagnosis, chemotherapy (yes/no), radiation field, and follow-up duration. Lymphocyte TL (LTL) was measured by telomere flow-FISH cytometry using blood samples banked at a mean of 38.9 years (cases), 39.2 years (controls). Genetic variation in telomere genes was assessed by whole genome sequencing. Point estimates for LTL <10th percentile were determined for cases and controls. Results: Cases had shorter median LTL than controls in three out of four leukocyte subsets. Cases were more likely to have NK cell LTL <10th percentile (P = 0.01), and 2.8-fold more likely to have naïve T-cell LTL <10th percentile than controls (CI, 1.07-8.78). Five out of 15 cases with a rare indel or missense variant had naïve T-cell LTL <10th percentile, compared with one out of eight controls. Conclusions: Long-term survivors have shorter than expected LTL, a finding that is more pronounced among survivors with thyroid SMN.