The objective of this study was to correlate radiologic changes to arthroscopic findings in patients with osteoarthritis of the tibiofemoral joint. The study group consisted of 105 patients, 52 men and 53 women, with a mean age of 51 +/- 9.3 years (range, 39-83 years), with symptomatic tibiofemoral joint osteoarthritis (TFOA). X-rays were graded according to Kellgren and Lawrence grading (KLG) by 2 independent board-certified radiologists. All patients had arthroscopy, and findings were recorded and analyzed per Outerbridge classification (OC). Approval of the Institutional Research and Ethical Committee was obtained. Statistical analysis was done using the Statistical Package for the Social Sciences, version 17 (SPSS Inc., Chicago, IL, USA). In men, the correlation of KLG to OC was more than 75% in grade II and joints appeared as grade IV, but with grades I and III, it was 66% and 57% (P = 0.05). In men in KLG groups I and III, the joints appeared to have milder affliction, but arthroscopy revealed otherwise. In women, the only correlation was of KLG grade IV to OC grade IV. In women, joint affliction was more severe than was predicted by the KLG (P = 0.01). This study shows that radiologic lagging occurs behind arthroscopic findings, especially in women and in apparent early arthritis in roentgenograms (lower radiologic grades). X-rays of the knee per se might give a false impression of a milder disease.