Magnetic resonance imaging in idiopathic retroperitoneal fibrosis: measurement of T1 relaxation time. Magnetic resonance imaging at 0.08 Tesla was performed in nine patients with proven idiopathic retroperitoneal fibrosis. A total of 11 scans was performed. Three patients were scanned before diagnosis; one of these also had two follow-up scans. A further six patients were scanned a variable time after diagnosis and treatment. On each scan, a periaortic soft-tissue mass was readily identified, the distribution corresponding to that seen on computed tomography. There was no difference in the mean T1 relaxation time of the mass between patients scanned before diagnosis and those scanned after treatment. However, the patient followed with serial scans showed a progressive reduction in the T1 value of the mass with time. Comparison with results obtained in patients with lymphoma suggests that the T1 values in retroperitoneal fibrosis are lower than in lymphoma, particularly non-Hodgkin's lymphoma. 