Multiple myeloma (MM) is characterized by malignant plasma cell infiltration of the bone marrow. In extramedullary multiple myeloma (EMD), a subclone of these cells migrates out of the bone marrow. Out of 4 985 MM patients diagnosed between 2005 and 2017 in the Czech Republic, we analyzed 234 secondary EMD patients to clarify risk factors of secondary EMD development. We found younger age [<65 years; odds ratio (OR) 4Á38, 95% confidence interval (CI): 2Á46-7Á80, P < 0Á0001], high lactate dehydrogenase (LDH) levels (>5 lkat/l; OR 2Á07, 95% CI: 1Á51-2Á84, P < 0Á0001), extensive osteolytic activity (OR 2Á21, 95% CI: 1Á54-3Á15, P < 0Á001), and immunoglobulin A (IgA; OR 1Á53, 95% CI: 1Á11-2Á11, P = 0Á009) or the non-secretory type of MM (OR 2Á83; 95% CI: 1Á32-6Á04, P = 0Á007) at the time of MM diagnosis to be the main risk factors for secondary EMD development. Newly diagnosed MM (NDMM) patients with subsequent EMD had inferior median progression-free (PFS) and overall (OS) survival when compared to NDMM patients without future EMD [mPFS: 13Á8 months (95% CI: 11Á4-16Á3) vs 18Á8 months (95% CI: 17Á7-19Á9), P = 0Á006; mOS: 26Á7 months (95% CI: 18Á1-35Á4) vs 58Á7 months (95% CI: 54Á8-62Á6), P < 0Á001]. We found that NDMM patients with specific risk factors associated with secondary EMD development have a more aggressive disease course before secondary EMD develops.