Background: Immunodeficiency is a shared feature of B cell malignancies. The risk of infections and their prognostic significance after diagnosis are well characterized, but, conversely, less is known about prediagnostic infections in these domains. Methods: In matched case-control analyzes, using Danish nationwide registers, we assessed the rate of prediagnostic infections in chronic lymphocytic leukemia (CLL), diffuse large B cell lymphoma (DLBCL), multiple myeloma (MM), follicular lymphoma (FL), marginal zone lymphoma (MZL), and lymphoplasmacytic lymphoma (LPL). Survival analyzes of data from clinical registers were then used to determine the effect of infections in the year preceding diagnosis on overall survival. To yield results for as many patients as possible, antimicrobial prescriptions were used as surrogates for infections. Results: The nationwide and clinical registers comprised 30,389 patients, accumulating 213,649 antimicrobial prescriptions, and 18,560 patients accumulating 107,268 prescriptions, respectively. The relative risk of infections was increased up to 15 years prior to diagnosis of malignancy and markedly increased in the year just prior to diagnosis. More than two antimicrobials within one year prior to diagnosis were associated with significantly shorter overall survival, independently of known prognostic factors. Conclusion: Patients with B cell-derived malignancies exhibit marked immunodeficiency several years prior to diagnosis such that different disease subtypes demonstrate both overlapping and distinct trends in infection risk preceding diagnosis. Moreover, multiple infections within the year preceding diagnosis are independently associated with shorter overall survival for all the examined malignancies. (Figure presented.)