Risk factors for brain metastasis in small cell lung cancer were studied in 260 patients without brain metastasis at presentation who were treated on 5 protocols. The first three protocols offered elective brain irradiation (EBI) to all available patients after 2 or 3 courses of chemotherapy (early EBI), whereas the other two offered it after 5 or 6 courses of chemotherapy (late EBI). The overall incidence of brain metastasis was higher in the late EBI group than in the early EBI group (23.6% vs. 14.3%, p = 0.08), primarily due to higher incidence of brain metastasis developing before EBI in the former. There was a higher incidence of brain metastasis in patients without than in patients with bone marrow metastasis (21.4% vs. 6.8%, p = 0.04), in patients less than 60 years old than in patients 60 years or older (23.4% vs. 13.4%, p = 0.06), and in patients with than in patients without bone metastasis (30.2% vs. 16.8%, p = 0.07). Major risk factors for short brain metastasis-free survival were bone metastasis (p = 0.008), late EBI (p = 0.03), and failure to achieve complete remission after induction chemotherapy (p = 0.001) or as a best response (p = 0.0001). The early EBI group had a longer brain metastasis-free survival than the late EBI group, even among patients with bone metastasis (p = 0.02) or bone marrow metastasis (p = 0.05) and those who achieved complete remission after induction chemotherapy (p = 0.06) or as a best response (p = 0.05). These results indicate that timing of EBI is a critical factor in brain metastasis in patients with small cell lung cancer. There was no difference in overall survival between the two groups, however, even among patients who achieved complete remission as a best response.