From January 1970 through December 1981, 469 patients with histologically or cytologically proven adenocarcinoma (AC) (349) and large cell carcinoma (LC) (120) of the lung were seen at the Department of Radiation Oncology, Medical College of Wisconsin Affiliated Hospitals. One quarter (126/469) of these patients had brain metastasis: 48 patients presented with brain metastasis (AC 35/349 = 10% and LC 13/120 = 11%) and 78 patients subsequently developed brain metastasis (AC 61/314 = 19%, LC 17/107 = 16%). Those patients who received prophylactic cranial irradiation were excluded from this study. Brain was the dominant site of metastasis in 82 patients who received only cranial + thoracic irradiation; 37 patients (17 simultaneous, 20 metachronous) also required irradiation of other sites of metastasis. All 17 patients with LC, and 47/61 (77%) with AC who developed metachronous brain metastasis did so within one year. The cumulative probability of brain metastasis increased with survival to the levels predicted by autopsy studies. Therapeutic brain irradiation may result in long-term survival in patients with single organ brain metastasis. Three of 119 patients who underwent whole brain irradiation for metastasis, are alive at 60 (LC), 48 (AC) and 48 months (AC) after treatment. Since patients with AC and LC so frequently develop brain metastasis and the brain may be the only site of metastasis, prophylactic cranial irradiation may significantly reduce morbidity and mortality from these diseases.