The genetic predisposition to inclusion body myositis (IBM) is probably multifactorial. The deposition of the β-amyloid protein is a characteristic histological feature of both IBM and Alzheimer's disease (AD). The ϵ4 allele of apolipoprotein E (APO E) has been strongly associated with familial and late-onset AD. We therefore compared the APO E allele frequencies in a group of 14 patients with IBM with those in a group of patients with other inflammatory muscle diseases and in the general population. The frequency of the ϵ4 allele in IBM was increased (0.29) compared with that in patients with other inflammatory muscle diseases (0.15) and the general population (0.13) (p < 0.05). These data suggest that APO E genotype may be one of the factors involved in determining the predisposition to the development of IBM.