Purpose: to validate a bony landmark on the dorsal aspect of the patella for measurement of the interbone distance in the axial radiogram of the patello-femoral joint (PFJ); to assess the reproducibility of this radiogram and the minimal joint-space (MJS) width measurements in the medial and lateral compartments of the PFJ in this view; and to relate the MJS of the PFJ to MR-detected cartilage defects in the same joint Material and Methods: Fifty-seven individuals with chronic knee pain (aged 41–58 years, mean 50 years) were examined with an axial view of the PFJ in the standing position. the MJS was measured with a mm-graded ruler. on the same day, an MR examination was performed with proton density- and T2-weighted turbo spin-echo sequences on a 1.0 T imager. We noted the cartilage defects in the PFJ in axial and sagittal MR images, and the shape of the sub-chondral cortex of the medial and lateral articular surfaces of the patella in sagittal MR images Results and Conclusion: the bony landmark was found on all articular surfaces. the axial view of the PFJ and the MJS measurements were reproducible. an MJS of < 5 mm showed high specificity for MR-detected cartilage defects. Thus an MJS of 5 mm proved to be a limit in the diagnosis of joint-space narrowing in the PFJ in middle-aged individuals with chronic knee pain