Background/Objective: Multiple myeloma (MM) is an incurable, yet treatable, cancer of plasma cells. Due to recent improvements in treatment, people diagnosed with MM have been living longer, and other co-morbid conditions may be of increasing importance. This study examines temporal trends in specific causes of death among MM patients in Puerto Rico (PR) and United States (US). Methods: We analyzed primary cause of death among all incident MM cancer cases recorded in the Puerto Rico Central Cancer Registry (PRCCR) (n=3,018) and the US Surveillance, Epidemiology, and End Results Program (SEER) (n=67,733) between 1987-2013, overall and by follow-up time, age, and sex. We calculated the cumulative incidence of death due to seven selected causes and analyzed age-adjusted mortality trends by MM and other causes using joinpoint regression. Results: MM accounted for 71.7% and 71.3% of all reported deaths in PR and US, respectively, among people diagnosed with MM. In PR, the proportion of patients that died from MM decreased with increasing follow-up time since diagnosis (72.3% of deaths with ≤2 years vs 65.6% with >5 years of follow-up) and the proportion of patients who died from circulatory (4.6% vs 9.0%) and respiratory system (3.7% vs 5.0%) diseases increased slightly. A similar trend of decreasing MM deaths with follow-up time was observed in the US (73.2% of deaths with ≤2 years vs 66.5% with >5 years of follow-up). Joinpoint regression showed a decreasing trend in MM mortality in the US (APC1987-2007=-2.8%, and APC2007-2013=-18.4%) and a similar, though somewhat weaker, trend in PR (APC1987-2013=-2.73). Conclusion: In both PR and the US, people diagnosed with MM are still more likely to die from MM than from another cause. However, a decrease in MM mortality is evident, particularly in more recent years, but this decrease is lower in Puerto Rico.