The amount of dose reduction on changing from low dose rate (LDR) brachytherapy to medium dose rate (MDR) or high dose rate (HDR) afterloading has been the subject of much debate. The magnitude of reduction depends, together with other possible factors, on two radiobiological parameters: the โฃ/โค ratio and the half-time of repair of the relevant tissues. In an attempt to extract these radiobiological parameters for the late rectal complications observed in our previously published clinical results four different schedules using MDR and one using LDR are analyzed. The percentage incidence of complications was a function of increasing biologically effective dose (BED), but would yield nonsense scattergrams if plotted against raw total dose. In addition, for three other published MDR series, three LDR series, and two HDR series, the incidence of rectal complications is plotted against BED to examine the predictive potential of using BED as the surrogate of total dose. Our own results were published in 1996, consisting of 102 patients treated at the LDR of 0.44 Gy/hr and 88 patients treated by four different schedules using an MDR of 1.6-1.7 Gy/hr. Follow-up is at least 3 years in all schedules. The linear quadratic formula including the ''g'' dose rate factor was used to analyze them, assuming exponential repair of the repairable beta term. First, multivariate and profilelikelihood analyses were carried out to obtain estimates of โฃ/โค and T 1 โ2 for rectal lateresponding tissues. Then graphs of incidence of rectal complications vs. BED were constructed, assuming โฃ/โค = 3 Gy and T 1 โ2 = 1.5 hr, values which had not been contradicted by the multivariate analysis. Graphs were drawn both for ''all grades including mild reactions'' (grades 1 + 2 + 3) and for ''serious'' complications (grade 3 in our system). In addition, other published cervical brachytherapy series were reviewed, with calculation of their BEDs if not published by the authors. It was necessary to review and compare their grading systems, so that ''mild and moderate'' (grades 1 and 2) could be contrasted with ''serious'' (grades 3 and 4 or 5 in various systems). Comparisons were made with other published results, including three LDR, three MDR, and two HDR series spanning from 1982 to 1997. The BEDs at which the incidence of rectal complications rose above the arbitrary level of 10% were compared for all three ranges of dose rate. The multivariate analysis gave estimates of โฃ/โค and T 1 โ2 which were not significantly different from 3 Gy and 1.5 hr, respectively, so these values were used to compute the BEDs for the subsequent comparisons. It was found that the graphs of incidence of rectal complications for ''all grades including mild'' agreed rather better between all series than might have been expected, within a provisional (10%) threshold BED of range 100-123 Gy 3 (60-74 Gy given as 2 Gy fractionated external beam or as LDR). The dose-response curves di-