1. Seventeen patients with diabetic retinopathy were studied before, and at various times following yttrium-90 implantation of the pituitary by means of fluorescence angiograms. 2. The studies demonstrated improvement in capillary abnormalities including microaneurysms, capillary dilatation and small areas of capillary nonperfusion following both partial and maximal pituitary ablation. 3. Study of microaneurysm turnover rates showed a significant de crease in the formation of new ones as compared with the rates of patients followed as controls to the pituitary implanted ones. Disappearance of existing microaneurysms was also accelerated, but this did not reach statistical significance. 4. Abnormalities in small (20–50 μ) arteries improved, while large arterial disease continued in spite of pituitary ablation. 5. New vessels often regressed but they only disappeared completely if the ablation was complete. 6. No retina returned to complete normality following pituitary ablation. 7. The most marked effect of pituitary ablation was the slowing down of the rate of formation of new microvascular lesions.

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