Encounter with infectious antigens has been proposed to initiate the cascade of events associated with progression from premalignancy (monoclonal gammopathy of undetermined significance, MGUS) to multiple myeloma (MM). We conducted a population-based case-control study to evaluate risk of developing MM associated with a personal history of various respiratory tracts infections occurring >1 year prior to MM. Inpatient (1977-1997) and outpatient (1994-1997) diagnoses were obtained for all MM patients (n=4,476) diagnosed in Denmark (1977-1997) and 16,727 matched controls. A personal history of pneumonia was associated with a 1.6-fold (95%CI 1.3-2.0) increased risk of MM; the elevated risk was restricted to 1-4.99 years prior to the diagnosis of MM (OR=1.7,95%CI 1.3-2.2). Individuals with two and three or more previous episodes of pneumonia had a 1.7-fold (95%CI 1.0-3.0; p=0.05) and a 1.5-fold (95%CI 0.6-3.9) elevated MM risk, respectively. Pneumonia could be a trigger to the development of MM or a manifestation of immune disturbances in late-stage MGUS.