AbstractEpidemiologic evidence is limited about associations between T2DM, metformin, and the risk of non‐Hodgkin's lymphoma (NHL). We aimed to examine associations between T2DM, metformin, and the risk of NHL in the Women's Health Initiative (WHI) Study. Information on T2DM status (diabetes status/types of antidiabetic drug use/diabetes duration) from study enrollment and during follow‐up were assessed. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated to evaluate associations of T2DM status with risks of overall NHL and its three major subtypes [diffuse large B‐cell lymphoma (DLBCL, n = 476), follicular lymphoma (FL, n = 301) and chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL, n = 136)] based on multivariable‐adjusted Cox proportional hazards models. During a median follow‐up of 18.86 years (range, 0.01‐25.13; SD ± 6.55), a total of 1637 women developed NHL among 147 885 postmenopausal women. Women with T2DM and with self‐reported oral medication use had 38% and 55% higher risk of DLBCL, respectively [multivariable‐adjusted model HR = 1.38, 95% CI (1.06‐1.81) and HR = 1.55, 95% CI (1.16‐2.06)] compared to the reference group (nondiabetics/untreated diabetes). Risks of NHL and DLBCL [multivariable‐adjusted model: HR = 1.28, 95% CI (1.06‐1.54) and HR = 1.56, 95% CI (1.13‐2.14), respectively] were significantly higher in associations with relatively short duration (≤7 years) of diabetes, compared to reference group. Additionally, an increased risk of DLBCL [HR = 1.76, 95% CI (1.13‐2.75)] was found in metformin users compared to the reference group. Postmenopausal women who had T2DM, who were oral antidiabetic drug users, especially metformin, and who had a shorter diabetes duration may have higher risks of DLBCL. Further well‐designed research is needed to confirm our findings.