Background: The purpose of this study was to determine the inter-and intra-observer reliability of the patellotrochlear index (PTI) on magnetic resonance images (MRI) in patients with patellofemoral pain. The correlation between the PTI measured on MRI and the modified Insall-Salvati (MIS) ratio measured on radiographs was also assessed. Methods: The PTI was assessed on MRI images and the MIS ratio on radiographs of 66 knees of 62 patients. Assessment was performed by two orthopaedic surgeons, one orthopaedic surgery registrar, two radiologists and one radiology registrar. Correlation coefficients, standard errors of measurement and limits of agreement were calculated for the PTI. To assess the association between the PTI and the MIS ratio, the Pearson's correlation coefficient was calculated. Results: The PTI showed good interobserver reliability (intraclass correlation coefficient (ICC) 0.79; 95% confidence interval (CI) 0.73-0.85) and excellent intra-observer reliability (ICC 0.90; 95% CI 0.89-0.91). The standard error of measurement was 0.05 and limits of agreement with the mean ± 0.09. A very weak and not significant correlation was found between the PTI and the MIS (r = 0.02; P = 0.77). Conclusions: The PTI showed good interobserver reliability and excellent intra-observer reliability. In order to conclude which measurement method of assessing patellar height is truly the most reliable, future studies should investigate agreement parameters (standard error of measurement, limits of agreement) besides solely correlation coefficients. We found a very weak correlation between the PTI and the MIS which suggests that at least one index has poor validity. Future validity studies on indices to assess patellar height are necessary.