Context: The increased use of diagnostic and therapeutic procedures that involve radiation raises concerns about radiation effects, particularly in children and the radiosensitive thyroid gland. Objectives: Evaluation of relative risk (RR) trends for thyroid radiation doses ,0.2 gray (Gy); evidence of a threshold dose; and possible modifiers of the dose-response, e.g., sex, age at exposure, time since exposure. Design and Setting: Pooled data from nine cohort studies of childhood external radiation exposure and thyroid cancer with individualized dose estimates, 1000 irradiated subjects or 10 thyroid cancer cases, with data limited to individuals receiving doses ,0.2 Gy. Participants: Cohorts included the following: childhood cancer survivors (n = 2); children treated for benign diseases (n = 6); and children who survived the atomic bombings in Japan (n = 1). There were 252 cases and 2,588,559 person-years in irradiated individuals and 142 cases and 1,865,957 personyears in nonirradiated individuals. Intervention: There were no interventions. Main Outcome Measure: Incident thyroid cancers. Results: For both ,0.2 and ,0.1 Gy, RRs increased with thyroid dose (P < 0.01), without significant departure from linearity (P = 0.77 and P = 0.66, respectively). Estimates of threshold dose ranged from 0.0 to 0.03 Gy, with an upper 95% confidence bound of 0.04 Gy. The increasing dose-response trend persisted .45 years after exposure, was greater at younger age at exposure and younger attained age, and was similar by sex and number of treatments. Conclusions: Our analyses reaffirmed linearity of the dose response as the most plausible relationship for "as low as reasonably achievable" assessments for pediatric low-dose radiationassociated thyroid cancer risk.