The foveal photopic threshold for Landolt ring discrimination and the paramacular scotopic threshold has been monocularly measured in 27 diabetic patients, 30 to 50 years old, without funduscopic retinal alterations, and compared with similar thresholds collected from 17 healthy age matched subjects. About 0.3 log threshold elevation for both photopic and scotopic functions were documented in the diabetic group, suggesting a functional impairment before visible retinopathy occurred. The threshold loss was not correlated with the diabetes longevity.