This is a retrospective analysis of the long-term local control in 4128 patients with non-disseminated nasopharyngeal carcinoma treated solely by megavoltage irradiation during the years 1976-1985. The T-stage distribution according to Ho's classification was T1 37%, T2 14% and T3 49%. Different fractionation schedules had been employed at different periods, and the median dose to the primary target was equivalent to 65 Gy by time dose fractionation calculation. In 8% (344) of patients the tumour failed to regress completely after the basic course, but 89% (148/167) of those suitable for salvage with additional irradiation eventually attained complete local remission. The cumulative incidence of local failure was 24% (5% persistence, 19% recurrence). The 10-year actuarial local failure-free survival was 67%. While patients with T2 and T3a tumours achieved local control comparable to T1, those with T3c-d had the poorest control (with highest incidence of persistence and advanced recurrence). T-stage adjusted analyses suggested a significant trend of dose-response: the odds ratios for local failure were 1.16 and 1.86, respectively, when patients given 60-63 Gy and 55-59 Gy were compared with those given 64 Gy or above (p value = 0.0018). Patients treated during 1981-1985 achieved higher local failure-free survival than those treated during 1976-1980 (75% versus 70% at 5 years, p value = 0.0013). The possible attributes are studied, and ways for future optimization of treatment discussed.