In this study the solid cancer mortality data in the Techa River Cohort in the Southern Urals region of Russia was analyzed. The cohort received protracted exposure in the 1950s due to the releases of radioactive materials from the Mayak plutonium complex. The Extended Techa River Cohort includes 29,849 people who resided along the Techa River between 1950 and 1960 and were followed from January 1, 1950 through December 31, 1999. The analysis was done within the framework of the biologically based two-stage clonal expansion (TSCE) model. It was found that about 2.6% of the 1854 solid cancer deaths (excluding 18 bone cancer cases) could be related to radiation exposure. At age 63, which is the mean age for solid cancer deaths, the excess relative risk (ERR) and excess absolute risk (EAR) were found to be 0.76 Gy(-1) (95% CI 0.23; 1.29) and 33.0 (10(4) PY Gy)(-1) (95% CI 9.8; 52.6), respectively. These risk estimates are consistent with earlier excess relative risk analyses for the same cohort. The change in the ERR with age was investigated in detail, and an increase in risk with attained age was observed. Furthermore, the data were tested for possible signs of genomic instability, and it was found that the data could be described equally well by a model incorporating effects of genomic instability. Results from the TSCE models indicated that radiation received at older ages might have stronger biological effects than exposure at younger ages.