Our understanding of cancer risk from ionizing radiation is largely based on studies of populations exposed at high dose and high dose rates. Less certain is the magnitude of cancer risk from protracted, low-dose and low-dose-rate radiation exposure. We estimated the dose-response relationship for solid cancer mortality in a cohort of 29,730 individuals who lived along the Techa River between 1950 and 1960. This population was exposed to both external γ radiation and internal ⁹⁰Sr, ¹³⁷Cs and other radionuclides after the release of radioactive waste into the river by the Mayak Radiochemical Plant. The analysis utilized the latest individualized doses from the Techa River Dosimetry System (TRDS) 2009. We estimated excess relative risks (ERRs) per Gy for solid cancer mortality using Poisson regression methods with 95% confidence intervals (CIs) and P values based on likelihood ratio tests. Between 1950 and 2007, there were 2,303 solid cancer deaths. The linear ERR/Gy = 0.61 (95%; CI 0.04–1.27), P = 0.03. It is estimated that approximately 2% (49.7) of solid cancers deaths were associated with the radiation exposure. Our results, based on 2,303 solid cancer deaths and more than 50 years of follow-up, support an increased risk of solid cancer mortality following protracted radiation exposure from the Techa River contamination. The wide confidence interval of our estimate reflects the challenges of quantifying and describing the shape of the dose-response relationship in the low dose range. Nevertheless, the risk estimates provide important information concerning health risks from whole-body radiation exposure that can occur from accidents that result in wide-scale environmental contamination.