This study validated a brief measure of fatigue in rheumatoid arthritis (RA), the Functional Assessment of Chronic Illness Therapy (FACIT) Fatigue Scale. The FACIT Fatigue was tested along with measures previously validated in RA: the Multidimensional Assessment of Fatigue (MAF) and Medical Outcomes Study Short-Form 36 (SF-36) Vitality. The sample included 636 patients with RA enrolled in a 24 week double blind, randomized clinical trial (RCT) of adalimumab versus placebo. The FACIT Fatigue showed good internal consistency (alpha = 0.86 to 0.87), strong association with SF-36 Vitality (r = 0.73 to 0.84) and MAF (r = -0.84 to -0.88), and the ability to differentiate patients according to clinical change using the American College of Rheumatology (ACR) response criteria (ACR 20/50/70). Psychometric performance of the FACIT Fatigue scale was comparable to that of the other 2 fatigue measures. A minimally important difference in FACIT Fatigue change score of 3-4 points was confirmed in a separate sample of 271 patients with RA enrolled in a second double blind RCT of adalimumab versus placebo. The FACIT Fatigue is a brief, valid measure for monitoring this important symptom and its effects on patients with RA.