This study was undertaken to investigate the possibility of increasing the dose per fraction or increasing the number of fractions to account for treatment delays occurring during radiotherapy treatments for highly proliferative tumours. The linear quadratic model with time was used to determine the difference in biological effective dose (BED) for the original schedule and the schedule including a treatment delay. Tables of extra fractions and extra dose per fraction required to account for a number of possible delays have been determined. It has been shown that for tumours with very short potential doubling times it is best to deliver the extra dose as an increase in dose per fraction rather than an increase in the number of fractions, while for tumours with moderately short potential doubling times (above 7 days) the reverse is true. The equivalent uninterrupted schedules, which would have delivered the same effects to the tumour, have also been determined.