Mutations in BEST1 , encoding bestrophin-1 (Best1), cause Best vitelliform macular dystrophy (BVMD), a dominantly inherited macular degeneration characterized by a diminished electrooculogram light peak (LP), lipofuscin in retinal pigment epithelial cells (RPE), and fluid- and debris-filled retinal detachments. To understand the pathogenesis of BVMD we generated knock-in mice carrying the BVMD-causing mutation W93C in Best1. Both Best1 ^+/W93C and Best1 ^W93C/W93C mice had normal ERG a- and b-waves, but exhibited an altered LP luminance response reminiscent of that observed in BVMD patients. Morphological analysis identified fluid- and debris-filled retinal detachments in mice as young as 6 months of age. By 18–24 months of age Best1 ^+/W93C and Best1 ^W93C/W93C mice exhibited enhanced accumulation of lipofuscin in the RPE, and a significant deposition of debris composed of unphagocytosed photoreceptor outer segments and lipofuscin granules in the subretinal space. Although Best1 is thought to function as a Ca ²⁺ -activated Cl ⁻ channel, RPE cells from Best1 ^W93C mice exhibited normal Cl ⁻ conductances. We have previously shown that Best1 ^−/− mice exhibit increased [Ca ²⁺ ] _i in response to ATP stimulation. However, ATP-stimulated changes in [Ca ²⁺ ] _i in RPE cells from Best1 ^+/W93C and Best1 ^W93C/W93C mice were suppressed relative to Best1 ^+/+ littermates. Based on these data we conclude that mice carrying the Best1 ^W93C mutation are a valid model for BVMD. Furthermore, these data suggest that BVMD is not because of Best1 deficiency, as the phenotypes of Best1 ^+/W93C and Best1 ^W93C/W93C mice are distinct from that of Best1 ^−/− mice with regard to lipofuscin accumulation, and changes in the LP and ATP Ca ²⁺ responses.