Sequential thermoradiotherapy was given to 24 patients with locally advanced breast carcinoma. The outcome of treatment was analyzed in relation to the thermal dose as well as clinical stage, tumor volume, vascular density and necrotic fraction prior to treatment. Complete or partial response was seen in all patients. Three of the patients showed local recurrence following treatment. One of the recurrences was due to a geographic miss of the radiation therapy whereas the other two recurrences were 'true' local treatment failures. Local treatment failure could not be attributed to an advanced clinical stage, large tumor masses or poor tumor heating. The recurrent tumors had a low vascular density and significant necrotic fraction prior to treatment, suggesting that treatment failure was due to the presence of chronic hypoxia. The radiotherapeutic problem of treatment resistance caused by chronically hypoxic cells is probably not eliminated with sequential thermoradiotherapy.