For over two years, we monitored the visual functions and ocular findings of two siblings with Batten disease. At the initial visit, the older patient was in the advanced stage of the disease, manifesting symptoms similar to those previously reported by many authors. The younger patient, on the other hand, was in the early stage of the disease at the time of the first visit. In the next 8 months, however, the visual acuity of the younger patient decreased rapidly, from 1.0 to below 0.1. Although the electroretinogram (ERG) showed a normal a-wave and a reduced b-wave when visual acuity was normal, repeated recordings of the ERG at each visit revealed rapid reduction of the b-wave. This finding seems to confirm the animal data, indicating that the primary lesion of the retina was in the inner layers. To our knowledge, this is the first report on these phenomena which may be unique to the deteriorating visual system in Batten disease and may facilitate early diagnosis.