Neurodegeneration in the retina precedes damage to the substantia nigra and striatum in Parkinson disease (PD), as demonstrated in a recently published study using a rotenone-induced rat model of PD. 20 days after the rotenone insult, in vivo imaging and optical coherence tomography revealed increased retinal ganglion cell apoptosis and swelling of the retinal layers. These findings could pave the way for strategies to diagnose PD early in the disease course.