Twelve published clinical results of radical radiotherapy of head and neck cancer have been reviewed, seven of them with fresh multivariate analyses, to determine the magnitude of time factors relating local control to overall time. In all but two of the data sets a significant loss of local control was observed with prolongation. The median rate of loss was 14% in only 1 week, the range 3 to 25%. This corresponds to a median loss of 26% in 2 weeks (5-42%). These results are comparable with other, less detailed information. Whether these significant losses are due to proliferation of tumor cells or to other causes such as physician selection, it is clear that modest prolongation is associated with a lower chance of local control.