Intracavitary irradiation therapy for carcinoma of the uterine cervix used with high or low dose rate irradiation is fractionated in Japan. The optimal treatment dose is determined according to the biological effect on both diseased and healthy tissues. The equations of modified NSD and TDF biological equivalents were recalculated from Arai's clinical data, which were used to examine the optimal time-dose-fractionation relationship for high and low dose rate intracavitary irradiation on squamous cell carcinoma of the cervix uteri. The optimal time-dose-fractionation relationship at point A is expressed as follows: D = NSD N0.26 T0.06 where NSD is 17.75 get for high dose rate and 31.78 get for low dose rate. TDF = K n d1.47 x-0.09 where K is 1.46 for high dose rate and 0.62 for low dose rate. The range of the optimal total dose to point A given by one fraction per week was 30.7 Gy for 4 fractions and 38.3 Gy for 8 fractions in high dose rate irradiation. In the case of low dose rate irradiation, the optimal total dose given by one fraction per week and the dose rate of 75.0 cGy/h was 55.0 Gy for 4 fractions. The maximum dose difference between our result and Arai's was about +/- 10%. The dose modification ratio for high dose rate and low dose rate is 1.79.