Purpose: To describe a five-grade classification of ectopia lentis in Marfan syndrome (MFS) and to evaluate the positive predictive value of the early grades of ectopia lentis. Methods: We prospectively included MFS patients and their healthy relatives. The anterior segment examination was classified into grades 0 to 5, and we studied the sensitivity, specificity, and positive predictive value of ectopia lentis in this classification. Results: Seventy-four MFS patients and thirty-six healthy controls were examined. In the MFS group, grades 1, 2, 3, and 4 were present in 15, 24, 17, and 7 patients, respectively, whereas 11 patients in this group did not present ectopia lentis. In the control group, grades 0 and 1 were observed in 30 and 6 individuals, respectively. Sensitivity to ectopia lentis of at least grade 2 was 64.9%, with 100% specificity, whereas sensitivity to ectopia lentis of at least grade 1 was 85.1%, with 83.3% specificity. The positive predictive value of ectopia lentis that was greater than or equal to grade 2 was 100%, whereas that of ectopia lentis greater than or equal to grade 1 was 91.3%. Conclusions: High positive predictive values s were found to be associated with grades 2 and higher of the five-grade classification of ectopia lentis. This classification should help to harmonize clinical practices for this major feature of MFS.