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To evaluate and characterize long-term clinical changes in keratoconus cases not requiring surgical treatment because of the absence of clinically significant visual degradation and to design a predictive model for the progression rate.This retrospective longitudinal study enrolled 68 nonsurgically treated keratoconic eyes of 46 patients (age range, 18–66 years), with a follow-up period of 6 years ± 12 months. Visual, refractive, topographic, and aberrometric changes were evaluated, detecting the abnormal levels of longitudinal changes in each clinical variable analyzed (percentiles 10% or 90%). Subsequently, the eyes were stratified into progression and no-progression groups based on the different variables evaluated, analyzing baseline differences between them. A progression index (PI%) was defined, and its relationship with baseline data was modeled using multiple linear regression analysis.Significant differences were detected in more quantity of baseline parameters between progression and no-progression groups for changes in sphere, spherical-like and coma-like root mean square, central mean keratometry (KM3mm), corneal asphericity (Q80mm), and central astigmatism (Ast3mm). The progression index (PI%) was defined using these variables, obtaining a mean value of 24.39 ± 51.09% and a median of 1.85%. This parameter was significantly related to the baseline sphere, K23 mm (steepest central keratometry), and Q80 mm (P < 0.001, R2 = 0.429).More pronounced progression seems to be present in myopic eyes with central localized steepening but normal mid-periphery flattening. A new index has been defined to characterize and predict the level of progression in nonsurgically treated keratoconus in a medium–long term.