MR images in 36 hips with documented avascular necrosis and 80 hips without evidence of joint disease were studied to determine the amount and appearance of fluid in the joint. All MRI examinations were done on a 1.5-T machine and included coronal images made with relative T2 weighting (repetition times = 2000-2500 msec, echo delays = 60-100 msec). The amount of joint fluid, which had an intense signal higher than fat, was graded from 0 to 3 and analyzed with respect to the patient's age and radiographic stage of avascular necrosis. Joint fluid was seen in 84% of presumed normal hips. Only four (5%) of 80 had enough fluid to surround the femoral neck (grade 2), and none had sufficient fluid to distend the joint capsule (grade 3). In comparison, 21 (58%) of 36 hips with avascular necrosis had grade-2 or grade-3 effusions (p less than 0.005), and some fluid was seen in all. Grade-3 effusions were seen in seven (50%) of 14 hips with flattening of the femoral head, compared with only one (5%) of 20 in which the femoral contour was normal. It is concluded that small amounts of fluid are present in both normal hips and those with avascular necrosis. In avascular necrosis, increased joint fluid may be present before radiographic abnormalities occur, but it is greatest after there is flattening of the femoral head. MRI is a highly sensitive method for detecting fluid in the hip joint.