BACKGROUND AND PURPOSE Because opinions on the significance of repopulation during radiotherapy of T1 laryngeal cancer vary, we have estimated the effective rate of tumour cell repopulation during radiotherapy in patients with T1 laryngeal cancer. MATERIALS AND METHODS One hundred seventeen consecutive patients with T1 laryngeal cancer were treated from 1982 to 1993 by radical radiotherapy alone either as continuous (n = 28) or split-course treatment (n = 89). The logit method of the linear-quadratic formula for local control at 3 years was used to examine the effect of treatment time on local control. The analysis was made for all patients to obtain a wide range of overall treatment times. RESULTS The 3-year overall survival rate was 76% and the 3-year local control rate was 85% (range 82-88%). The local control rates were 95% (range 94-96%) for the continuous and 81% (range 75-91%) for the split-course therapy groups, respectively. The results showed a mean Dprolif value at the steepest part of the response versus time curve of 0.48 Gy/day for local control at 3 years although this was not statistically significant. The trade-off of dose required to compensate for a 1 week increase in treatment time for local control at the 90% level achieved at 3 years was calculated to be 3.5 Gy. CONCLUSIONS The present results suggest that repopulation should be taken into account even when treating small T1 laryngeal cancer and that protraction of the overall treatment time should be avoided.