To determine the correlation between Magnetic Resonance Imaging (MRI) and clinical examination of the knee after an acute injury, and to see to what extent MRI affected the planned treatment, we examined 90 consecutive patients in a prospective study, clinically and with an extremity Magnetic Resonance (MR) scanner. The number of meniscal lesions, bone bruises and osteochondral lesions found on MRI was significantly higher than the clinical examinations indicated. Despite this, the treatment was only changed in 6 cases. In no case did MRI prevent a planned arthroscopy. MRI may reveal many clinically silent changes in the knee, also after minor injuries. The significance of these MRI findings must await long-term follow-up.