Colour vision and dark adaptation were studied in 60 diabetic patients treated with photocoagulation. An acquired colour vision defect was found in 50% and a defective dark adaptation in 77% of the patients. Both colour vision and dark adaptation were defective in 47% of the patients. Colour vision defect only was found in 3% and dark adaptation defect only in 30% of the patients. Neither of these defects was found in 20% of the patients. The Farnsworth-Munsell 100-hue test showed a blue-yellow axis in 24 of the 30 colour defective patients. In the patients with an impaired dark adaptation, the range of the cone and rod threshold elevations from the normal upper limit was from 0.2 to 1.4 log units. The Farnsworth-Munsell 100-hue test scores showed a significant positive correlation to the values of both cone and rod thresholds.