||CONTEXT: Hepatitis C virus (HCV) infection causes liver cancer and cirrhosis and may also increase the risk of other tumors, particularly hematopoietic malignancies and thyroid cancer. Previous studies have been too small to adequately assess these risks. OBJECTIVE: To test the hypothesis that HCV infection is associated with increased risk for hematological malignancies, related lymphoproliferative disorders, and thyroid cancer. DESIGN, SETTING, AND PATIENTS: A retrospective cohort study of users of US Veterans Affairs health care facilities from 1997-2004, which included 146,394 patients infected with HCV who had at least 2 visits with a diagnostic code for HCV infection, and 572,293 patients uninfected with HCV. To assemble the HCV-uninfected cohort, we randomly selected up to 4 patients per patient infected with HCV from all veterans who matched on age, sex, and baseline visit date and type (inpatient or outpatient). Individuals with human immunodeficiency virus were excluded. MAIN OUTCOME MEASURES: Risks of hematopoietic malignancies, related lymphoproliferative precursor diseases, and thyroid cancer, adjusting for selection factors, race, era of military service, and use of medical services. RESULTS: The mean (SD) age of the patients was 52 (8) years, and 97% were men. Risks for non-Hodgkin lymphoma (n = 1359), Waldenström macroglobulinemia (n = 165), and cryoglobulinemia (n = 551) were increased with HCV infection (adjusted hazard ratio [HR], 1.28; 95% confidence interval [CI], 1.12-1.45; adjusted HR, 2.76; 95% CI, 2.01-3.79; and adjusted HR, 3.98; 95% CI, 3.36-4.72; respectively). We found no significantly increased risk for other hematological malignancies. Although thyroiditis risk was slightly increased, risk for thyroid cancer (n = 320) was not (adjusted HR, 0.72; 95% CI, 0.52-0.99). Adjusted P values for non-Hodgkin lymphoma, Waldenström macroglobulinemia, cryoglobulinemia, and thyroiditis were all <.0038, the Bonferroni threshold for statistical significance considering multiple comparisons. CONCLUSIONS: Hepatitis C virus infection confers a 20% to 30% increased risk of non-Hodgkin lymphoma overall, and a 3-fold higher risk of Waldenström macroglobulinemia, a low-grade lymphoma. Risks were also increased for cryoglobulinemia. These results support an etiological role for HCV in causing lymphoproliferation and causing non-Hodgkin lymphoma.