Int. J. Environ. Res. Public Health 2009, 6 185 In 1967 a systematic program to define fixed cohorts of Techa River residents was initiated [26], about 26,500 people who lived in villages along the Techa River during the period of the highest releases (1949-1952) and for whom residence records were available being enrolled in a cohort now known as the Original Techa River Cohort (OTRC). Over the years, about 5,000 people born prior to 1950 who had moved to one of the riverside villages between 1953 and 1960 were added to the database (“late entrants”). In 1998, these two groups were merged to form the Extended Techa River Cohort (ETRC). In September, 2001, the ETRC included 30,136 people, of whom 4,953 were late entrants [27]. A third cohort consists of about 30,000 children born to exposed parents since 1950: the Techa River Offspring Cohort (TROC). The complete registry of exposed populations living around Mayak PA also contains information on exposed persons in the EURT area. Altogether, the database contains personal data for about 80,000 individuals, 17,000 of whom are from the EURT area. The catchment area for the OTRC included territory of five rural districts in the Chelyabinsk and Kurgan districts through which the Techa River flows, these districts including all of the contaminated villages, as well as all villages where people were evacuated to from the most contaminated villages. Death certificates and other documents about cohort members could be routinely obtained from these areas and the database also contains information such as residence history and results of medical and dosimetric examinations of a population including individuals who have received unusually high doses, but at low to moderate dose rates. It is important to note that attaining the required data for such a large population group is not without difficulty: published results for cohort members will include some cases of incomplete follow-up due to migration, missing records and unknown causes of death. The limitations and future plans for cohort analyses are further discussed in Kossenko et al. [27]. 2.1. Dose Reconstruction The average effective dose equivalents (representing dose accumulation over 25 years) for persons in the OTRC have been estimated to be from 0.074 Sv to 1.4 Sv, with average absorbed doses to bone marrow estimated as from 0.176 to 1.64 Gy [28]. Initial calculations indicated that individual bone surface doses and external doses could have exceeded 2 Gy [29]. Dose reconstruction methods are undergoing continuing refinement with the development of the Techa River Dosimetry System (TRDS-1996 and 2000) [30]. Reconstruction of the source term and amounts of radioactive material discharged into the Techa River has been very important for dose reconstruction. Operational data like the history of dam construction and watercourse changes have been studied as well as environmental monitoring data to obtain the best possible estimate of the exposure of inhabitants along the river. The reconstructed source term is assumed to be about 100 PBq (1017 Bq) of fission products released into the Techa River: most (~98 %) is assumed to have been discharged between the years 1950 and 1951 [1, 31]. However, environmental monitoring data were not available for the first three years of activities at the Mayak PA such that modeling methods have been used to estimate the missing information about discharges. Mokrov [32-33] recently published new assessments about the Techa source term for the early years of discharges of radioactivity, based on hitherto unavailable data from the Mayak PA. The main conclusions were that the accepted discharge inventory of 100 PBq was probably an underestimation of the total amounts of radioactivity discharged from Mayak during this