Objectives: Early-deafened, late-implanted adolescents and adults constitute a unique group of cochlear implant (CI) users, showing a large variability in outcomes. The current systematic review aimed to determine which preimplantation factors are relevant in predicting postoperative outcomes in this patient group. Design: A systematic search for studies published between 2000 and September 2017 was performed in five electronic databases (PubMed, Embase, the Cochrane library, CINAHL, and PsycInfo). Prognostic studies that assessed the relation between patient-related factors and CI outcomes in early-deafened but late-implanted adolescent and adult CI users were included. Study quality was assessed with the Quality In Prognosis Studies (QUIPS) tool. Results: The systematic search and subsequent full-text evaluation identified 13 studies that had a clear prognostic study goal. Eight out of these 13 studies had a high risk of bias for at least one of the five QUIPS domains. Analysis of the outcomes identified that communication mode (in childhood), preoperative speech intelligibility, and preoperative speech recognition scores were significantly related to speech perception outcomes for this group of CI users. A number of additional factors considered worth for further investigation were also identified. Conclusions: The analysis of the studies assessing the effect of a number of patient-related factors on outcome with CI in early-deafened adolescents and adults identified three potential prognostic factors and a number of additional factors to be incorporated in future research. More high-quality prognosis studies in the investigated patient population are still needed.