Although ionizing radiation is a known carcinogen, the long-term risk from relatively higher-dose diagnostic procedures during childhood is less well known. We evaluated this risk indirectly by assessing thyroid cancer incidence in a cohort treated with “lower-dose” chest radiotherapy more than 55 years ago. Between 2004 and 2008, we re-surveyed a population-based cohort of subjects treated with radiation for an enlarged thymus during infancy between 1926 and 1957 and their unexposed siblings. Thyroid cancer occurred in 50 irradiated subjects (mean thyroid dose, 1.29 Gy) and in 13 nonirradiated siblings during 334,347 person-years of follow-up. After adjusting for attained age, Jewish religion, sex and history of goiter, the rate ratio for thyroid cancer was 5.6 (95% CI: 3.1–10.8). The adjusted excess relative risk per gray was 3.2 (95% CI: 1.5–6.6). The adjusted excess absolute risk per gray was 2.2 cases (95% CI: 1.4–3.2) per 10,000 person-years. Cumulative thyroid cancer incidence remains elevated in this cohort after a median 57.5 years of follow-up and is dose-dependent. Although the incidence appeared to decrease after 40 years, increased risk remains a lifelong concern in those exposed to lower doses of medical radiation during early childhood.