Analysis of 801 multiple myeloma patients revealed that 8.7% developed atrial fibrillation after autologous stem cell transplantation, with median onset occurring just 13 days post-procedure. Age above 65 years nearly doubled the risk (HR: 1.88), while prior paroxysmal AF episodes increased risk more than six-fold (HR: 6.19), and obesity doubled it (HR: 2.00). This finding fills a critical knowledge gap in cardio-oncology, where atrial fibrillation represents one of the most common cardiovascular complications in cancer care yet remains poorly characterized in specific treatment contexts. The early median onset suggests that the conditioning regimen and stem cell infusion process itself may trigger arrhythmias through acute physiological stress, electrolyte shifts, or inflammatory responses. For multiple myeloma patients—who often undergo transplantation as standard care—this 8.7% incidence rate provides concrete risk stratification data that oncologists and cardiologists can use for pre-procedural counseling and monitoring protocols. The identification of modifiable factors like obesity alongside non-modifiable predictors suggests opportunities for targeted prevention strategies. While the study's retrospective design limits causal inference, the large cohort and systematic electrocardiographic surveillance offer robust incidence estimates that previous smaller studies couldn't provide.