Epidemiologic data on inclusion body myositis (IBM) are scarce, and possibl
y biased, because they are derived from larger neuromuscular centers. The p
resent nationwide collaborative cross-sectional study, which culminated on
July 1, 1999, resulted in identification of 76 patients with IBM and the es
tablishment of a prevalence of 4.9 patients with IBM per million inhabitant
s in the Netherlands. Several discrepancies suggest that this may be an und
erestimation. The most frequently identified pitfall in diagnosing IBM was
an erroneous diagnosis of polymyositis or motor neuron disease.