The aim of this study was to compare the visual functions of a group of young insulin dependent diabetes mellitus patients (IDDMs), (n = 42) with an age and sex matched control group (n = 24). Examinations were carried out every 3 months for 2 years. There were no significant differences in visual acuity between IDDMs and control subjects. The IDDMs made significantly more errors with the desaturated D15 compared with the control group (P < 0.05). The contrast sensitivity for the IDDMs was lower at each spatial frequency compared with the control group, being significantly different from 3 c/deg and above (P < 0.05). A negative correlation was found between HbA1 and contrast sensitivity at 6 and 12 c/deg; as the HbA1 increased the contrast sensitivity decreased (P < 0.05). As the blood glucose level decreased, the colour vision deteriorated (P < 0.05). There was no significant difference between the visual performance of those IDDMs with retinopathy (n = 5) compared to those without.