Purpose: To describe the disease course in patients with vitelliform macular dystrophy (VMD) with a Best1 mutation and to determine the association between Best1 genotype and visual prognosis. Design: Consecutive case series. Participants: Fifty-three patients with VMD with Best1 mutations from 27 Dutch families, aged 11 to 87 years. Methods: Best-corrected visual acuity (VA), fundus appearance, and Arden ratio on the electro-oculogram (EOG) during clinical follow-up were assessed from medical records. Mutation analysis of the Best1 gene was performed on DNA samples using denaturing high-pressure liquid chromatography and direct sequencing. Main Outcome Measures: Cumulative lifetime risk of visual decline below 0.5, 0.3, and 0.1 for the entire group and stratified for genotype. Results: Median age of onset of visual symptoms was 33 years (range: 2-78). The cumulative risk of VA below 0.5 (20/40) was 50% at 55 years and 75% at 66 years. The cumulative risk of decline less than 0.3 (20/63)was 50% by age 66 years and 75% by age 74 years. Two patients progressed to VA less than 0.1 (20/200). Fourteen different mutations were found. Most patients (96%) had missense mutations; the Thr6Pro, Ala10Val, and Tyr227Asn mutations were most common. Visual decline was significantly faster in patients with an Ala10Val mutation than either the Thr6Pro or the Tyr227Asn mutation (P=0.001). Conclusions: Age of onset of visual symptoms varies greatly among patients with VMD. All patients show a gradual decrease in VA, and most progress to visual impairment at a relatively late age. Our data suggest a phenotype-genotype correlation, because the Ala10Val mutation has a more rapid disease progression than other common mutations. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article. Ophthalmology 2010; 117: 1415-1422 (C) 2010 by the American Academy of Ophthalmology.