The purpose of this study was to investigate the treatment response in lateral epicondylitis (tennis elbow) by MRI. Magnetic resonance imaging was obtained in 30 patients with clinical symptoms of lateral epicondylitis of the elbow using T1-, T2- and T2-weighted fat-saturated (FS) sequences. The patients were randomised to either i.m. corticosteroid injection (n=16) or immobilisation in a wrist splint (n=14). Magnetic resonance imaging of the elbow was performed on a 1.5-T MR system at baseline and after 6 weeks. The extensor carpi radialis (ECRB) tendon, the radial collateral ligament, lateral humerus epicondyle at tendon insertion site, joint fluid and signal intensity changes within brachio-radialis and anconeus muscles were evaluated on the MR units workstation before and after 6 weeks of treatment. The MRI was performed once in 22 healthy controls for comparison and all images evaluated by an investigator blinded to the clinical status of the subjects. The MR images showed thickening with separation of the ECRB tendon from the radial collateral ligament and abnormal signal change in 25 of the 30 patients on the T1-weighted sequences at inclusion. The signal intensity of the ECRB tendon was increased in 24 of the 30 patients with lateral epicondylitis of the elbow on the T2-weighted FS sequences. In the patients there were no associations between pathologically signal intensity within the ECRB tendon on T1- and T2-weighted sequences and the degree of self-reported pain (Dumbells test) at inclusion. In general, the MRI changes persisted in the patients at follow-up after 6 weeks despite clinical remission. The increased signal intensity within the extensor tendon is indicative of lateral epicondylitis humeri. The changes in signal intensity and morphology of ECRB tendon seem to be chronic and may persist despite clinical improvement.