Posterior subcapsular cataracts (PSC) are associated with hereditary retinal dystrophy in the Royal College of Surgeons (RCS) rat model and with human retinitis pigmentosa. The relationship of lens and retinal pathology has never been explained. Previous studies of pink-eyed RCS rats aged 2.5 to 11 months had shown an incidence of cataract of 24% when observed by the unaided eye and 60% by direct ophthalmoscopy, while 40% of rats were considered to have clear lenses. Unlike the retinal degeneration, which appeared in all homozygous animals, cataract seemed not to be predictably associated with therdymutation. To test this further, we studied the lenses of rats of different ages with a diagnostic slitlamp. We confirmed that by 8 to 15 months of age, rats fed a diet containing recommended concentrations of all known nutrients for rodents developed cataracts with an incidence of 23% when observed by unaided eye. In addition, opacities were seen in 74% with the indirect ophthalmoscope and 20 D lens; but 100% had at least a“ugar grain”type PSC by slitlamp. The slitlamp-detectable cataract was first seen in some animals by 49 days, and by 56 days all rats examined had bilateral PSC. This is an age at which the rod photoreceptors have degenerated. We concluded that slitlamp-detectable PSC are predictably associated with the retinal dystrophy of therdymutation. The RCS rat model may be relevant to a type of retinal degeneration having a constant association of cataract.