We report the clinical features of and MRI findings in transient osteoporosis of the hip during pregnancy. The study population consisted of 4 patients with a mean age of 33 years. The mean gestational age at onset was 31 weeks (range: 27 to 35 weeks). The main symptoms consisted of a weight-bearing pain in the hip and gait disturbance. The pain occured suddenly and was of unknown cause and became severe within 2 to 3 weeks. X-ray examinations showed diffuse osteoporosis in the femoral head and neck. Moreover in 3 patients, similar lesions were also found in the lumbar spine or the knee. MRI obtained from 3 patients revealed a mottled low-signal lesion extending from the femoral head and neck on T1-weighted images and a high-signal lesion in the bone marrow suggesting edema on T2-weighted images. Mild elevation of C- reactive protein was shown in 2 patients. Conservative treatments with the limitation of weight bearing and bed rest were performed for all patients, and nonsteroidal anti-inflammatory drugs were given to 3 patients. The hip pain began to decline from 8 to 14 weeks after the onset, and completely disappeared from 14 to 24 weeks. X-ray examinations showed that osteoporotic lesions tended to improve at 10 to 14 weeks, on MRI, a high-signal lesion suggesting bone marrow edema resolved together with relief of the pain. No recurrence was found in any patients at mean follow-up of 70.8 months.