Men with diabetes are especially prone to neuronal and endothelial disease and are afflicted with erectile dysfunction (ED) at a much higher incidence and prevalence than normal men, with a consequent reduction in quality of life. ED in diabetes results from combinations of impairments from nearly every step in the production of a penile erection. These include the failed transmission of neural signals to and from the spinal cord due to neuropathy resulting in reduced neural nitric oxide (NO) delivery to cavernosal smooth muscle, impaired sinusoidal endothelial cell NO release because of endothelial dysfunction, reduced arterial and arteriolar inflow due to peripheral vascular disease, and failure of relaxation of the corpora from glycation of the elastic fibers. The evaluation of men with diabetes and ED is similar to that in nondiabetic men. It includes assessment of gonadal, neural, arterial, and on occasion, venous function, along with marital and psychological status. Although treatment options for diabetic men with ED have fortunately expanded, the risk factors for ED are the same as those for cardiovascular disease; a flagging penis should raise a red flag of warning to evaluate and treat the risk factors for coronary, cerebral, and peripheral vascular disease, which are common in persons, of either sex, afflicted by diabetes.