The dose-time factors in the external beam treatment of 473 patients with squamous cell carcinoma of the pharyngeal wall, vocal cord, pyriform sinus or supraglottic larynx were considered. The effect of overall treatment time on the tumor response was quantified by estimating the increment in dose per day needed to achieve a constant rate of local control, that is, the dose required to counterbalance the effect of growth of the tumor during irradiation. The estimated increment in isoeffect dose per day varies between sites, however, the increments, although estimated with considerable uncertainty, are in general larger than 1 Gy per day. These estimates are consistent with accelerated tumor clonogen repopulation during irradiation.