Cancer-associated retinopathy (CAR) should be suspected in patients who present with visual symptoms such as rapid unexplained visual loss and seeing shimmering lights, with an abnormal ERG. Electronegative ERG responses are not exclusive to melanoma-associated retinopathy (MAR) but may be seen in CAR as well. We describe a patient with CAR who presented with an electronegative ERG. A 67-year old woman, who presented with complaints of seeing shimmering lights, underwent an extensive ophthalmological and electrophysiological examination. Best-corrected visual acuity was 7/10 in the right and 9/10 in the left eye. Goldmann visual fields showed relative central scotomata and concentric narrowing. Slit-lamp and fundus examination were normal as was fluorescein angiography. Rod-specific ERG responses were severely reduced, with electronegative maximal combined rod-cone responses and delayed cone-responses with normal amplitudes. Melanoma-associated retinopathy was suspected. Extensive dermatological and internal evaluation eventually revealed an oat-cell carcinoma in the right lung. The patient died of pneumonia 2 years after presentation.