The magnetic resonance (MR) characteristics of the normal thymus in 18 patients were compared with computed tomographic (CT) findings in 13 of the 18. Patients ranged in age from 5 to 77 years. The thymus was visible in all patients and differed from subcutaneous fat in hydrogen density; the average thymus to fat hydrogen density ratio was 0.60. Although the T1 relaxation times of the thymus (mean = 703 msec) were much longer than those of fat (mean = 287 msec) in patients under 30 years of age, this difference decreased with age. The T2 relaxation times of the thymus were similar to those of fat and did not change with age. The thymus appeared thicker on MR images than on CT scans in patients older than 20 years. MR may be better than CT in distinguishing between thymus replaced by fat and mediastinal fat.