It is suspected that radiation retinopathy is more likely to develop in an eye with preexisting diabetic retinopathy than in a normal eye. However, there is only one report of this occurring, at a radiation dose of 4500 rads. We present a woman with minimal diabetic retinopathy who had breast carcinoma which was treated with chemotherapy but metastasised to the choroid. Within nine months of external beam radiation (3000 rads in fractions of 200 rads) a fulminant retinopathy evolved in that eye, while the non-radiated eye showed no change. The histopathology of radiation and diabetic retinopathy and causes for possible synergism are discussed. As this case report shows, radiation oncologists and ophthalmologists need to be aware of the risk that patients with minimal diabetic retinopathy who have undergone chemotherapy may suffer a dramatic visual loss from radiation therapy despite a radiation dose which is considered adequate, safe, and properly fractionated.