OBJECTIVE: The mortality risk among patients with diabetes is increasingly severe, yet the relationship between obesity and mortality risk in these patients remains controversial. This study evaluated the Conicity index (C-index), an indicator of abdominal obesity, to determine its value in predicting cardiovascular disease (CVD) and all-cause mortality in patients with diabetes. METHODS: This cross-sectional study utilized NHANES 1999-2018 data. Patients were grouped into quartiles based on the C-index. The relationship between the C-index and mortality risk was assessed using Cox proportional hazards regression models and restricted cubic spline (RCS) analysis. RESULTS: A total of 7694 patients with diabetes were included in the study. The obesity rate was 55.7 %, with an average follow-up duration of 88 months. During this period, 588 CVD deaths and 2094 all-cause deaths occurred. Higher C-index quartiles were associated with increased mortality risks, with hazard ratios for all-cause mortality ranging from 1.00 to 2.29 and for CVD mortality from 1.00 to 2.23. Unadjusted RCS analysis showed a linear positive correlation between the C-index and mortality risks. After adjusting for confounding factors, a non-linear positive correlation was observed between the C-index and all-cause mortality risk, particularly when the C-index exceeded 1.40. Subgroup analysis revealed that the relationship between the C-index and all-cause mortality was more significant in men and nonobese patients. CONCLUSIONS: The C-index is a valuable predictor of mortality in patients with diabetes. A C-index above 1.40, being male, and being nonobese are associated with a more significant risk of all-cause mortality.