Inclusion body myositis (IBM), a sporadic inflammatory myopathy, is the most frequently occurring progressive myopathy in adults older than 55 years. It more commonly affects men and usually is clinically and pathologically distinguishable from dermatomyositis or polymyositis. Muscle biopsy specimens will display inflammation in virtually all cases of sporadic IBM, along with rimmed vacuoles accompanied by the accumulation of beta-amyloid and other substances similar to those found in the degenerating neurons of Alzheimer's disease. The similarities between IBM and other inflammatory myopathies may contribute to its low level of diagnosis by pathologists. The proper recognition of IBM is important because, unlike other inflammatory myopathies, IBM is unresponsive to anti-inflammatory drugs.