A prospective study involving 190 patients presenting with symptoms suggesting internal derangements of the knee was performed to demonstrate the likelihood of symptoms and signs produced by medial patellar plica to mimic those of medial meniscal lesions. Arthroscopic examination was done in each patient by an independent examiner after the establishment of clinical diagnoses. Only patients with isolated medial patellar plica (Group A, n = 54) and isolated medial meniscal lesion (Group B, n = 19) were further assessed. Although isolated medial patellar plica can produce symptoms and signs similar to medial meniscal lesion, statistical analysis clearly demonstrated that giving way, locking, medial joint line tenderness and positive McMurray test were more commonly detected in Group B (p < 0.05). The sensitivity of detecting either one or more than one symptom or sign included in this study was higher in group B (89.5%), but a positive symptom or sign was more predictive in Group A (39.2%). Considering the variation of prevalence of medial patellar plica in other regions, the results from this study indicate that giving way, locking, clicking, medial joint line tenderness and positive McMurray test would be more predictive of medial patellar plica and thus clinical diagnosis of medial meniscal lesion might be affected if the prevalence of isolated medial patellar plica exceeded 16.6 per cent.