A 27-year-old man, while serving in the armed forces, presented with complaints of right knee pain of 3 months duration. The patient reported pain localized on the lateral side of the right knee, especially during long-distance walking and stair climbing, and a feeling of giving way. There was no history of recent or remote trauma to the knee joint. At that time, he presented with knee pain and instability consistent with anterior cruciate ligament (ACL) insufficiency. During physical examination, the diagnosis of nail-patella syndrome (NPS) was made, but magnetic resonance imaging (MRI) of both knees revealed intact ACLs. The reported case presents NPS as a potential nontraumatic cause of anterior knee instability. In our patient, a discrepancy between the clinical and imaging findings was present. The initial clinical examination was consistent with ACL rupture or aplasia and this led to MRI evaluation of the knee, which showed an intact ACL, contrary to the initial diagnosis. It should be noted that anterior instability has been described after patellectomy and could be attributed to complete absence of the patella, which is rare in NPS.