Forty-two patients with histologically confirmed inoperable small (oat) cell lung cancer were treated with local tumor irradiation (approximately 4,000 cGy) combined with "one-time" irradiation of the upper and lower half of the body. Twenty-five patients (Group I) received the irradiation in lateral position first to the upper half-body, and six weeks later to the lower half-body, the midplane dose averaging 880 cGy (not corrected for lung tissue). In 17 patients, lethal pneumonitis occurred. The mean time of survival was 8.2 months for 22 patients having the disease limited to one thorax-side and 4.0 months for three patients having distant metastases, stated by conventional clinical and X ray investigations. Seventeen patients (Group II) received the two half-body irradiation treatments through anterior-posterior fields. The average dose had been reduced to 800 cGy (uncorrected) and was given on one day in two fractions (600 and 200 cGy, separated by an interval of 5 hours). Furthermore, the forearms and the lower legs had been left outside the fields. Only one patient showed pneumonitis. The mean time of survival was 14.3 months for 12 patients having the disease limited to one thorax-side and 6.6 months for five patients having distant metastases. In both groups, serious reactions of the bone marrow were not observed. In Group II, leucocytes and lymphocytes had reached their original values three months after the end of irradiation. For all 42 patients, X ray films showed complete regression of the primary tumor. In 12 out of 35 autopsy specimens, the primary tumor could not be detected histologically, but only five of these were free of metastases. Presently, additional irradiation of the two halves of the body with lower and fractionated doses are being tested in order to achieve the further reduction of tumor cell numbers.