CONTEXT: Fibroids are noncancerous uterine tumors potentially associated with cardiovascular risk factors. OBJECTIVE: We aimed to examine prospectively associations of glucose, insulin, sex hormone-binding globulin (SHBG), and diabetes with incidence of fibroid diagnoses in midlife. METHODS: Participants in the Study of Women's Health Across the Nation (SWAN) cohort (n = 2570) reported fibroid diagnoses at enrollment (1996-1997) and 13 follow-up visits (1996-2013). At all visits, we measured glucose, insulin, and SHBG in fasting blood samples and calculated homeostatic model assessment for insulin resistance (HOMA-IR). Diabetes was defined using glucose levels, self-reported diabetes, or diabetes medication use. We used discrete-time survival models to estimate hazard ratios (HR) and 95% CI for associations of time-varying biomarkers and diabetes with incident fibroid diagnoses, adjusted for demographics and health care utilization. We also evaluated effect modification by menopausal status. RESULTS: At baseline, 2.7% of participants (n = 70) were using diabetes medication. Time-varying glucose, insulin, HOMA-IR, and SHBG were not associated with fibroid diagnosis. However, diabetes was associated with a 28% lower incidence of fibroid diagnosis (adjusted HR 0.72, 95% CI 0.44, 1.17), driven by participants using metformin (adjusted HR 0.49, 95% CI 0.21, 1.12), though precision was limited. After stratification by menopausal status, higher HOMA-IR and insulin were associated with greater incidence of fibroid diagnosis during premenopause but not perimenopause, while the inverse association between diabetes and fibroids was strongest during perimenopause. CONCLUSION: The effect of diabetes and biomarkers on fibroids may vary by menopausal status. Fibroid risk may increase with insulin resistance and decrease with diabetes treatment.