To date, the lesions of the popliteal system, which are responsible for the so-called postero-lateral knee instability, are still an imperfectly known chapter of knee conditions. We retrospectively analyzed 215 consecutive MR examinations performed May-August 1991 to evaluate MR capabilities in correctly depicting both the normal anatomy and the pathologic conditions of the popliteal system. Using our standard protocols for knee study and two different magnets (0.064 and 0.5 T), we identified 23 lesions of the popliteus and of its meniscal and peritoneal insertions, and 17 lesions involving also other capsular and ligamentous structures of the knee. Coronal T1-weighted images were the best ones to correctly identify normal knee anatomy, whereas sagittal T2-weighted images were more sensitive in detecting popliteal lesions. Our results had clinical or surgical confirmation in all cases.