To study the incidence of rimmed basophilic vacuoles (RBV) and 15-21 nm filamentous inclusions in neuromuscular disorders, other than inclusion body myositis (IBM) and to determine the diagnostic value of RBV quantitation in the differential diagnosis of IBM, we reviewed 1600 muscle biopsies for RBV and 750 biopsies for filamentous inclusions. The number of RBV-positive fibers per 10 mm2--the RBV-fiber density--was determined. The incidence of RBV in non-IBM biopsies was 8.8 per 1000. Major diagnostic categories were neurogenic disorders (n = 7) and limb girdle muscular dystrophies (LGMD) (n = 3). In IBM (n = 7) the RBV-fiber density ranged from 10.4 to 63.1 and was significantly higher than in neurogenic disorders (0.9-4.4) and LGMD (1.1-2.7). The highest value was found in rigid spine syndrome (205.8). Filamentous inclusions were seen in 2.7 per 1000 non-IBM biopsies, including familial oculopharyngeal muscular dystrophy with distal myopathy (OPMD-DM), rigid spine syndrome, acid maltase deficiency and amyloid neuropathy. RBV and filamentous inclusions coexisted in rigid spine syndrome and in familial OPMD-DM. RBV, as well as filamentous inclusions, has a very low incidence in non-IBM neuromuscular disorders; the RBV-fiber density may help to discriminate neurogenic disorders and LGMD from IBM.