When analysis of results of radiotherapy for nearly 500 patients with oropharyngeal cancer showed evidence for rapid tumor regrowth during extensions of treatment from about 5 weeks to about 8 weeks, we searched the literature on radiotherapy for head and neck cancer to determine whether it revealed similar evidence of accelerated tumor regrowth. Estimates of doses to achieve local control in 50% of cases (TCD50) were made from published local control rates, and the dependence of these doses on overall treatment duration was evaluated. In parallel, published scattergrams were analyzed to estimate the rate of tumor regrowth over the period of 4-10 weeks from initiation of therapy. Both analyses suggested that, on average, clonogen repopulation in squamous cell carcinomas of the head and neck accelerates only after a lag period of the order of 4 +/- 1 weeks after initiation of radiotherapy and that a dose increment of about 0.6 Gy per day is required to compensate for this repopulation. Such a dose increment is consistent with a 4-day clonogen doubling rate, compared with a median of about 60 days in published reports of unperturbed tumor growth rates. The values presented here are average values for a large number of patients: it is necessary, not only to verify the results of these retrospective analyses in prospective studies, but also to develop methods to predict the time of onset and rate of accelerated tumor clonogen repopulation in the individual patient.