To identify magnetic resonance (MR) imaging characteristics of normal patellar tendons and those affected by tendinitis, the authors evaluated MR images obtained in 10 healthy volunteers, in 50 patients who underwent MR imaging for evaluation of knee structures other than the patellar tendon, in 11 patients with patellar tendinitis, and in two athletes with patellar tendon injuries. Normal tendons had uniformly low signal intensity on T1-, T2-, and proton-density-weighted images and displayed distinct margins, and the anteroposterior (AP) diameter slightly increased proximally to distally. It was concluded that the AP diameter of a normal tendon, in its proximal portion, should not exceed 7 mm. In patellar tendinitis, the tendon showed increased signal intensity on T1-, T2-, and proton-density-weighted images and increased AP diameter proximally. The margins of affected tendons were indistinct, especially posterior to the thickened segment. In all groups studied, women had thicker proximal tendons than did men.