The object of the study was to quantify the absorbed doses to urinary bladder using MIRDOSE3 (medical internal radiation dose package program) depending on drinking water after giving radioiodine dose to thyroid cancer patients. Twenty-nine female thyroid cancer patients (aged 40-60 years, mean 50 years) were selected. The therapeutic doses ranged from 3700 to 7400 MBq of I-131. The radioiodine uptake was measured at 1 cm distance from three organs (previously marked), the thyroid, thigh and stomach, by using a calibrated Eberline ESP-2 GM counter, with a special arrangement of each patient. Urine samples were collected every 12 h for first 72 h, and then every 24 h for the next 96 h. The individual biological half-life of excreted urine was calculated using individual effective half-life. Absorbed doses were calculated for an adult female phantom using the dynamic bladder model of MIRDOSE3 program in two phases: firstly, for different voiding intervals; and secondly, depending on individual drinking water. An average of 85% of the total dose passed through the urinary tract within the first 72 h, with a biological half-life of 28.5 +/- 0.747 h, and 9% for the next 96 h with a biological half life of 118.43 +/- 0.645 h. The voiding interval shows great impact on total absorbed dose to bladder and water supplementation needs to be intensified to reduce absorbed doses to bladder wall for the first 3 days. ((C) 2003 Lippincott Williams Wilkins).