It is very important to evaluate the uncertainties in individual monitoring for internal exposure of workers. The uncertainties in estimated body burdens of 137Cs with the JAERI whole-body counter were investigated using Monte Carlo simulation and measurements. It was found that the uncertainties of estimated body burdens with the whole-body counter are strongly dependent on various sources of uncertainty, such as radioactivity distribution within the body and counting statistics and that the 137Cs body burden assessed from the result of the whole-body count can be within +/- 60% in error.