In order to assess the behaviour of radioactivity (specifically (131)I concentrations in wastewater and sludge at sewage treatment plant) discharged into municipal sewage by therapy patients receiving treatment, the use of radiopharmaceuticals in four hospitals was quantitatively evaluated and the patients were classified by their residence. (131)I concentrations were estimated using a simple assumption model for radioiodine treatments of 94 patients, and measured by HPGe in inflow points, respectively. A good agreement was found between (131)I concentrations estimated with the model and those measured in the inflow points at the sewage treatment plant.