Despite being perhaps the most fundamental distinction among human beings, the impact of sex on outcomes in multiple myeloma (MM) is unknown. Given that men are more likely to develop MM compared to women, and an analysis of all cancers using the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program suggested possible worse MM survival for men than women, 1 a comprehensive evaluation of sex differences in survival in MM is warranted. In an analysis of 1 960 MM patients enrolled in a phase III clinical trial, sex was associated with overall survival (OS) on univariate analysis (median OS 44Á8 months female vs 49Á9 months male, P = 0Á02) but was not significant in multivariate analysis. Similarly, an analysis of the Myeloma XI trial found no sex differences in progression-free survival (PFS) or OS. 3 In this report, we describe baseline characteristics and outcomes of patients with MM by sex using two large population-based datasets: the SEER dataset and the Multiple Myeloma Research Foundation (MMRF) CoMMpass dataset with clinical annotation. We used MM data for the period 2000-2017 from SEER 18 that covers about 27Á8% of the United States population. We identified MM patients (n = 78,351) using the third edition of the International Classification of Diseases for Oncology (ICD-O-3) code 9732. We also obtained data from the MMRF CoMMpass study (NCT01454297), a prospective observational study of newly diagnosed MM patients initiated in 2011. Data relevant to treatment outcomes of 1 143 MM patients were collected, including pre-treatment demographics, International Staging System (ISS), baseline MM parameters, cytogenetics, induction regimen, autologous stem cell transplant (ASCT) and maintenance therapy use, PFS and OS. We defined OS as time from diagnosis until death from any cause. PFS was not available in SEER. In the MMRF data, PFS was defined as the time from the date of diagnosis until progression, death or the date of last follow-up. Survival curves were constructed using the Kaplan-Meier method and compared with the log-rank test. To examine the association between sex and outcomes, we first used Cox proportional hazards models to calculate age-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for PFS and OS. In all models, males served as the reference. Age at diagnosis as a continuous variable and categorical variable (<55, 55-69, 70+) generated similar findings. For SEER, basic multivariate models included age, race and diagnosis year (Table SI). For MMRF, basic multivariate models included Fig 1. Kaplan-Meier survival curves for MM by sex. (A) Overall survival of SEER patients. (B) Overall survival of MMRF patients. (C) Progression-free survival of MMRF patients.