Background: ¹³¹I-metaiodobenzylguanidine (MIBG) remains one of the most active agents for neuroblastoma. The clinical and biologic predictors of response to MIBG therapy have not been systematically characterized in a recent era trial. Methods: Patients 1-30 years were enrolled on the randomized phase 2 trial of MIBG vs. MIBG/vincristine/irinotecan vs. MIBG/vorinostat for relapsed/refractory neuroblastoma (NANT2011-01, NCT02035137) between 2014-2019. Data were compared between those who had an objective response (partial response or better using NANT response criteria) to MIBG after the first cycle and those who did not according to clinical features (age at study enrollment; sex; response to prior therapy; prior receipt of MIBG therapy; bone marrow involvement at study enrollment; and measurable disease status at study enrollment) and MYCN status. Univariate analyses were performed using odds ratio and Fisher exact tests. Multivariate logistic regression analysis was used to identify predictors of response to MIBG therapy while controlling for key confounders. Results: 105 response-evaluable patients were included in the analytic cohort. Across the 3 study arms, 20% (21/105) had an objective response to the treatment. Only measurable disease status was statistically significantly associated with response to therapy, with higher rates of response among patients without measurable disease at study enrollment (30% vs. 13%, p = 0.049). Response to prior therapy, sex, MYCN status, and measurable disease status showed univariate odds ratios of 2 or greater (Table). Only measurable disease status remained statistically significant in the multivariate analysis (p = 0.043, Table). Conclusions: Patients without measurable disease have a higher rate of objective responses to MIBG therapy. Understanding these differences based upon predictors of response may help to inform stratification methods for future MIBG clinical trials.