What are the causes of multiple myeloma?

Unknown/MGUS: The exact cause of multiple myeloma is not known. Myeloma is almost always (?Always) preceded by monoclonal gammopathy of undetermined significance (mgus). Risks: age, male gender, black race; radiation, benzene/petroleum exposure laboratory studies can help predict risk of conversion from mgus to myeloma.

No simple answer: Multiple myeloma has been associated with ionizing radiation (atomic bomb level), herbicides, petroleum products, analin hair dyes, autoimmune disease but there is no clear cut association. There are a few families which show an inheritance pattern along with other lymphomas.