Subnormal corneal sensitivity has been reported previously in patients with diabetes mellitus, but is of uncertain etiology. In the present study, corneal sensitivity was measured in 18 experimentally diabetic dogs and 10 normal dogs of similar ages, using a Cochet-Bonnet aesthesiometer. Half of the 18 diabetic dogs were kept chronically hyperglycemic and glucosuric, whereas the prospective control of diabetes in the other half was intentionally better. Corneal sensitivity was found to be significantly less in the chronically glucosuric group than in the nondiabetic (p < 0.01) or well-controlled diabetic dogs (p < 0.05). The impairment of corneal sensitivity correlated significantly with diabetes duration only in the poorly controlled group (r=0.753, p<0.02). Poorly controlled diabetes of long-standing duration causes a significant reduction in corneal sensitivity which may make these corneas more susceptible to damage, particularly with the stress of ocular surgery or contact lens usage.