This study used the methods of the Farnsworth-Munsell 100-hue test (FM-100), heterochromatic brightness matching (HBM) and wavelength discrimination to test the sensitivity and colour vision of 20 juvenile diabetics with no (16) or very mild (4) retinopathy. Their results were compared to an age-matched control group. The FM-100 results showed a significant increase in error scores throughout the spectrum in comparison to the controls. This deterioration in colour vision was confirmed in the results for the wavelength discrimination task, tested between 440 and 640 nm, where the just noticeable difference in colour was, in general, larger for the diabetic group than the control group. Only at 460 nm were the results of the diabetics similar to those of the controls. The diabetic group were also less sensitive than the control group in the HBM task between 480 and 600 nm. The results show that a deficit in sensitivity and colour vision occurs in diabetics before the onset of a clinically visible retinopathy.