Background: Patients with certain autoimmune diseases (AID) have an increased risk of developing diffuse large B-cell lymphoma (DLBCL). However, the occurrence of AID in patients with DLBCL as well as the impact of AID on outcome has not been extensively studied. The main purpose of this study was to establish the occurrence of AIDs in a population-based cohort of DLBCL patients and to compare outcomes in patients with or without AID treated with rituximab(R)-CHOP/CHOP-like treatment. We also aimed to analyse gender differences and the potential role of different AIDs on outcome and the frequency of treatment-associated neutropenic fever. Patients and methods: All adult patients treated 2000-2013 with R-CHOP/CHOP-like treatment for DLBCL in four counties of Sweden were included (n = 612). Lymphoma characteristics, outcome and the presence of AID were obtained through medical records. Results: The number of patients with AID was 106 (17.3%). Thyroid disease dominated (n = 33, 31.1%) followed by rheumatoid arthritis (RA) (n = 24, 22.6%). The proportion of AID was significantly higher in females (59/254, 23.2%) vs. in males (47/358, 13.1%) (p = .001). In the whole cohort there was no difference in event free survival (EFS) or overall survival (OS) between patients with or without AID. However, patients with an AID primarily mediated by B-cell responses (thyroid disorders excluded) had a worse OS (p = .037), which seemed to affect only women. The AID group more often had neutropenic fever after first treatment (16.0% vs 8.7%, p = .034) and those with neutropenic fever had a worse OS (p = .026) in Kaplan-Meier analyses. Conclusion: There is a high prevalence of AID among patients with DLBCL. AIDs categorized as primarily B-cell mediated (in this study mainly RA, systemic lupus erythematosus and Sjögren's syndrome) may be associated with inferior OS. AID patients may be more prone to neutropenic fever compared to patients without concomitant AID.