To analyse the association between body mass index (BMI) and all-cause mortality in a 5-year follow-up study with Spanish type 2 diabetes mellitus (T2DM) patients, seeking gender differences. 3443 T2DM outpatients were studied. At baseline and annually, patients were subjected to anamnesis, a physical examination, and biochemical tests. Data about demographic and clinical characteristics was also recorded, as was the treatment each patient had been prescribed. Mortality records were obtained from the Spanish National Institute of Statistics. Survival curves for BMI categories (Gehan-Wilcoxon test) and a multivariate Cox proportional hazard analysis were performed to identify adjusted Hazard Ratios (HRs) of mortality. Mortality rate was 26.38 cases per 1000patient-years (95% CI, 23.92–29.01), with higher rates in men (28.43 per 1000patient-years; 95% CI, 24.87–32.36) than in women (24.31 per 1000patient-years; 95% CI, 21.02–27.98) (p=0.079). Mortality rates according to BMI categories were: 56.7 (95% CI, 40.8–76.6), 28.4 (95% CI, 22.9–34.9), 24.8 (95% CI, 21.5–28.5), 21 (95% CI, 16.3–26.6) and 23.7 (95% CI, 14.3–37) per 1000person-years for participants with a BMI of <23, 23–26.8, 26.9–33.1, 33.2–39.4, and >39.4kg/m2, respectively. The BMI values associated with the highest all-cause mortality were <23kg/m2, but only in males HR: 2.78 (95% CI, 1.72–4.49; p<0.001), since in females this association was not significant HR: 1.14 (95% CI, 0.64–2.04; p=0.666) (reference category for BMI: 23.0–26.8kg/m2). Higher BMIs were not associated with higher mortality rates. In an outpatient T2DM Mediterranean population sample, low BMI predicted all-cause mortality only in males. •In a Spanish cohort of T2DM outpatients, mortality rate is 26.38/per 1000patient-years.•In males, the BMI value showing the highest all-cause mortality is <23kg/m2.•Higher BMIs are not associated with higher mortality rates in either males or females.