26 patients with unresectable locally recurrent adenocarcinoma of the rectum were treated with a mixed beam schedule. 40 Gy photons were delivered to the whole pelvis followed by a neutron boost of 6.6 or 10 Gy. Neutron therapy was carried out with a 14 MeV d-t generator (KARIN) using an isocentric are technique. Fluctuation in neutron dose rate during irradiation was monitored by a computer which controlled the gantry speed. All patients were followed clinically by CEA monitoring and CT-scans. In 18 patients positron-emission-tomography (PET) was used to verify the therapeutic efficacy. All patients were symptomatic with severe pain prior to therapy. After a mean follow-up interval of 12.8 months (range six to 26 months), the palliative effect in terms of pain relief was excellent in 22 patients in spite of the poor general condition of most patients and the large tumor extension. In four patients, further pain symptoms developed again after six to nine months due to renewed tumor progression. We observed proctitis at late side effects in one, enteritis in two and a fistula in one patient six to ten months after therapy. Changes in tumor glucose metabolism were monitored by serial PET examinations in all patients. The typical pattern observed by PET was a decrease in the F-18-Deoxyglucose (FDG) accumulation, approximately six weeks after onset of therapy.