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Every year, a monitoring of an internal contamination with ¹³⁷Cs is carried out in NRPI Prague. The monitoring is performed in whole-body counting laboratory on a reference group of 30 people (15 male and 15 female), mainly Prague inhabitants between the ages of 26 and 70 years. Based on those measurements in the year of 2009, the average activity of ¹³⁷Cs in an individual body has been estimated to be 40 Bq. As in previous years, national survey of internal contamination with ¹³⁷Cs has been carried out based on the urinary excretion of ¹³⁷Cs in 24-hours samples. Urine samples were collected from 26 males and 44 females representative of average population by their diet. Average activity of ¹³⁷Cs excreted in 24-hours urine sample is 0.16 Bq, resulting in a calculated average body content of ¹³⁷Cs (retention) of 27 Bq. Based on the results from the national survey, the committed effective dose from ¹³⁷Cs is estimated to be 0.96 Sv. Development in time of the average retention of ¹³⁷Cs obtained from the both data sets since the year 1986 is given in the picture.