Background: Certain medical conditions affect risk of non- Hodgkin lymphoma (NHL), but the full range of associations is unknown. We implemented a novel method ("medical condition- wide association study," MedWAS) to comprehensively evaluate medical risk factors for NHL documented in administrative health claims. Methods: Using Surveillance, Epidemiology, and End Results (SEER)-Medicare data, we conducted a case-control study comparing NHL cases [N = 52,691, age 66+ years, with five subtypes: chronic lymphocytic leukemia/small lymphocytic lymphoma, diffuse large B-cell lymphoma (DLBCL), follicular lymphoma, marginal zone lymphoma (MZL), T-cell lymphoma (TCL)] to controls (N = 200,000). We systematically screened for associations with 5,926 medical conditions documented in Medicare claims more than 1 year before selection. Results: Fifty-five conditions were variously associated with NHL. Examples include well-established associations of human immunodeficiency virus, solid organ transplantation, and hepatitis C virus with increased DLBCL risk (ORs 3.83, 4.27, and 1.74, respectively), and autoimmune conditions with DLBCL and MZL (e.g., ORs of 2.10 and 4.74, respectively, for Sjogren syndrome). Risks for all NHL subtypes were increased after diagnoses of nonmelanoma skin cancer (ORs 1.19-1.55), actinic keratosis (1.12-1.25), or hemolytic anemia (1.64-4.07). Nine additional skin conditions increased only TCL risk (ORs 2.20-4.12). Diabetes mellitus was associated with increased DLBCL risk (OR 1.09). Associations varied significantly across NHL subtypes for 49 conditions (89%). Conclusion: Using an exploratory method, we found numerous medical conditions associated with NHL risk, and many associations varied across NHL subtypes. Impact: These results point to etiologic heterogeneity among NHL subtypes. MedWAS is a new method for assessing the etiology of cancer and other diseases.