Sixty-six insulin-dependent diabetic patients with normal visual acuity were studied (132 eyes). The mean age was 27.7 years with a mean duration of diabetes mellitus of 8.7 years. Their mean electro-oculogram ratio, Arden grating and Munsell-Farnsworth scores were markedly abnormal (p much less than 0.00001). Background retinopathy was present in 53 eyes (40.2%), abnormal Arden grating scores (greater than 82) in 70 eyes (53%), abnormal Farnsworth-Munsell 100-hue score (greater than 100) in 88 eyes (56.7%), and 29 eyes (22%) had an electro-oculogram ratio less than 160. These abnormalities of retinal function did not correlate with retinopathy and were not secondary to it. Smoking caused a significant deterioration in contrast sensitivity.