Fractionated high dose rate versus low dose rate regimens for intracavitary brachytherapy of the cervix. I. General considerations based on radiobiology. Intracavitary brachytherapy at low dose rate (LDR), often with the addition of external-beam radiotherapy, has long been considered the treatment of choice for carcinoma of the cervix, maximizing acute damage in the treatment volume, whilst minimizing late effects. In recent years, primarily for reasons of convenience and cost, there has been a move towards treatments involving a few fractions at high dose rate (HDR). Using data from cells of human origin cultured in vitro, we make estimates of the doses that, delivered in 2-12 HDR fractions, produce tumour control and early effects equivalent to intracavitary treatments at LDR. We also show that, for situations where the normal-tissue dose responsible for late effects is significantly smaller than the tumour dose, HDR schemes may be devised which, while yielding early killing comparable with that of LDR, should not result in worse late effects. We suggest that this scenario probably applies to treatment of carcinoma of the cervix. 