Posttraumatic acute articular blocks of the knee may be caused by "mechanical" factors, such as the interposition of some osteochondral, meniscal or ligamentous fragments between condyles and tibial plateau or by non-mechanical ("functional") factors, such as the pain associated with capsular-ligamentous structure injuries or with intraosseous bruises involving the synovia. From January, 1994, to January, 1996, we examined 751 patients for posttraumatic knee injuries. The patients were submitted to MRI with a dedicated unit and we selected 86 of them who had an acute articular block as the major symptom. The block had a mechanical cause in 36 patients of the selected group (41.8%), as confirmed at arthroscopy, while the other 50 patients (58.2%) had a functional block. Fifteen of the latter patients had only intraosseous bruising, with no ligament or meniscus damage, while the other 35 had isolated or variously associated menisco-ligamentous injuries, as confirmed at arthroscopy. All the patients also had some intraosseous bruises which were clearly depicted at MRI. This study demonstrates that MRI is an extremely valuable tool in assessing the cause of articular blocks, allowing a prompt appropriate choice to be made between therapeutic arthroscopy and weight-free limb immobilization.