Multiple myeloma (MM) incidence and mortality are higher among African Americans (AAs) than among other population groups. The prevalence of obesity is also elevated among AAs, but few studies have examined risk of this cancer in relation to body size among AAs. We combined data from seven prospective cohorts tracking mortality among 239 597 AA adults and used Cox proportional hazards regression to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for death because of MM according to body mass index (BMI) at cohort entry, adjusted for age (as time-scale) and sex. Relative to those with normal BMIs (18.5-25 kg/m2), mortality increased monotonically as BMI increased, with hazard ratios reaching 1.43 (95% CI = 1.03 to 1.97) for BMIs of 35 kg/m2 or greater. The findings suggest that obesity is a risk factor for MM and a contributor to the elevated rates and rising incidence trends of MM among AAs in the United States.