The capabilities of MRI, radionuclide bone scanning, and X-ray CT in diagnosing avascular necrosis of the hip were compared in a controlled statistical study. Diagnostic ability was measured as the area under the receiver-operating-characteristic curve. Differences in efficacy among various case pools and the standard errors of those differences were calculated. MR was better than both other techniques over the entire case pool, with the difference between MR and radionuclide scanning exceeding 2 standard errors (p less than 0.01). In the subsample of patients including more early cases, MR was better than CT by greater than 2 standard errors and better than radionuclide scanning by over 3 standard errors. This is evidence that MR is the most sensitive imaging technique for the early diagnosis of avascular necrosis.