Leptin may be involved in the regulation of body weight, food intake, and energy expenditure. In view of a possible link between leptin concentrations and diabetes that has been suggested in obese rodents, we investigated the potential relationship between serum leptin concentrations and hyperglycaemia in French patients with morbid obesity. Fasting leptin concentrations were measured in 241 morbidly obese patients with various degrees of glucose tolerance in a cross‐sectional study. Fasting serum leptin concentrations did not differ between normoglycaemic (NG, 61.5±24.0 ng/ml) and glucose intolerant morbidly obese subjects (IGT, 56.5±18.5 ng/ml) and were slightly lower in those with controlled diabetes (55.1±30.3 ng/ml, P= 0.06 when compared to NG subjects). In contrast, leptin concentrations were 30% lower in patients with poorly controlled diabetes (43.0±22.2 ng/ml, P= 0.001 vs NG subjects). Leptin concentrations were negatively correlated with fasting glucose in all groups combined (ρ=− 0.24, P= 0.0001) and particularly in NIDDM subjects (ρ= 0.31, P= 0.0054). Although leptin concentrations were higher in women than in men, similar significant correlation with fasting glucose was found when females were analyzed separately. A positive correlation was found with BMI (ρ= 0.25, P= 0.0001) in all groups. Multivariate analysis revealed that fasting glucose was independently associated with serum leptin concentrations (F= 12.5, P= 0.0005). Sex, age, BMI, waist/hip ratio, fasting glucose and insulin, total cholesterol and triglycerides, tested in the model, explained 42% of the leptin variability in this population. Poorly controlled diabetes was accompanied by a significant reduction of serum leptin concentrations in morbidly obese subjects. We suggest that a relative leptin deficiency (lower than expected for the BMI) associated with insulin deficiency in this population might contribute to a vicious cycle maintaining (or even worsening) obesity itself and/or its metabolic complications.