To investigate serum leptin levels in patients with type 2 diabetes mellitus (T2DM) and the relationship between leptin levels and T2DM complications and prevalence.A total of 355 patients, 282 cases with T2DM and 73 normal controls, were recruited at 1st Medical Centre, Chinese PLA General Hospital (Beijing, China) between November 2013 and July 2014. Levels of serum leptin, biochemical markers and sexual hormones were measured, and clinical characteristics were retrieved through the electronic medical record system.Leptin levels in females were higher than that in males. Leptin levels in T2MD patients were positively correlated with body mass index, percent body fat, triglyceride, cystatin C homocysteine and salivary acid, and negatively correlated with glycosylated serum protein and glycosylated albumin levels. Leptin levels in males were positively correlated with systolic pressure and estradiol, and negatively correlated with testosterone and high density lipoprotein cholesterol. Sex (female) was positively correlated with the duration of disease. Leptin levels in T2DM patients with complications such as hypertension, diabetic nephropathy, diabetic peripheral neuropathy and coronary heart disease were higher than that in patients without such complications. Leptin levels in females with diabetic retinopathy and diabetic macroangiopathy were higher than that in patients without such complications, but there was no difference in males.Leptin has significant gender differences. Leptin levels are related to body mass index, percent body fat and sex hormone level in T2DM patients and may affect short-term blood glucose control in T2DM patients. Leptin levels are related to complications in patients with T2DM and affect the prevalence rates of complications.