Background: Patellofemoral cartilage deterioration and osteoarthritis are reported to be associated with recurrent patellar dislocation. However, the association between first-time traumatic patellar dislocation and cartilage deterioration is unknown. Purpose: The aim of this study was to assess long-term cartilage deterioration in the patellofemoral and tibiofemoral joint after conservatively treated traumatic lateral patellar dislocation. Study Design: Case series; Level of evidence, 4. Methods: Twenty patients (mean age, 25 years) who sustained first-time traumatic lateral patellar dislocation with no previous patellofemoral instability symptoms were initially scanned with 1.5-T magnetic resonance imaging (MRI). A follow-up 3-T MRI was conducted, on average, 8 years after first-time lateral patellar dislocation. Subjective instability symptoms and Knee injury and Osteoarthritis Outcome Score were also assessed. Results: In the primary MRI, patellofemoral cartilage injury was seen in 14 of 20 patients (70%). Most (14/15) of the injuries were seen in the patellar cartilage, especially at the medial facet. On the follow-up MRI, patellofemoral cartilage deterioration was visible for all patients. The central patella (P = .005) seemed especially prone to cartilage deterioration during the follow-up. Half of the patients (10/20) had grade 3-4 cartilage lesions in the patellofemoral joint in the follow-up MRI. In the primary MRI, only 1 patient exhibited tibiofemoral joint cartilage lesions, whereas at the time of follow-up, 10 of 20 patients exhibited tibiofemoral cartilage lesions. The majority of these lesions were considered clinically nonsignificant (International Cartilage Repair Society = 1) and were seen in the lateral compartment (6/10, 60%). Of the 14 patients (36%) with injury to the patellar cartilage, 5 reported subsequent instability of the patellofemoral joint, but this was not associated with more significant cartilage deterioration in the follow-up MRI compared with patients without redislocation. Conclusion: While recurrent lateral patellar dislocation is known to lead to degenerative process, a single first-time or infrequently recurring traumatic lateral patellar dislocation also seems to be associated with gradual cartilage deterioration. Traumatic lateral patellar dislocation might initiate gradual degeneration of the cartilage in the patellofemoral joint and can lead to the development of generalized knee osteoarthritis. Instability symptoms of the patellofemoral joint, however, were not related to the severity of the deterioration.