In a retrospective cohort of more than 4 million white and black male United States (US) veterans, we explored the role of specific prior autoimmune, infectious, inflammatory, and allergic disorders in the etiology of multiple myeloma (MM) and monoclonal gammopathy of undetermined significance (MGUS). Patients were selected from computerized inpatient discharge records at US Veterans Affairs hospitals. The analysis included 4641 patients (3040 white, 1601 black) and 2046 patients (1312 white; 734 black) with a discharge diagnosis of MM and MGUS, respectively. Using Poisson regression, we calculated age-adjusted relative risks (RRs) and 95% confidence intervals (CIs) for the relationship between MM, MGUS, and specific prior medical conditions. Significantly elevated risks of MM were associated with broad categories of autoimmune (RR, 1.15; 95% CI, 1.02-1.28), infectious (RR, 1.29; 95% CI, 1.20-1.38), and inflammatory disorders (RR, 1.18; 95% CI, 1.10-1.27) and specific prior autoimmune (polymyositis/dermatomyositis, systemic sclerosis, autoimmune hemolytic anemia, pernicious anemia, and ankylosing spondylitis), infectious (pneumonia, hepatitis, meningitis, septicemia, herpes zoster, and poliomyelitis), and inflammatory (glomerulonephritis, nephrotic syndrome, and osteoarthritis) disorders. Risks for MGUS were generally of similar magnitude. Our results indicate that various types of immune-mediated conditions might act as triggers for MM/MGUS development.