One hundred MRI examinations of normal subjects obtained at 0.5 T were studied in an effort to evaluate the claustrum and to establish a control group for patients with Wilson's disease. The claustrum was detectable unilaterally or bilaterally in 40 out of 100 subjects (40%) on spin-echo long TR (proton density and T2-weighted) MR images as a thin sheet of grey matter enclosed by low signal white matter of the external and extreme capsules. Spin-echo T1-weighted images were negative for the claustrum, however, it was identifiable in 12 out of 25 subjects (48%) studied utilizing the inversion recovery pulse sequence. In addition, eight patients with clinically established diagnoses of Wilson's disease were evaluated. The claustrum was normal (invisible) in four neurologically asymptomatic Wilson's disease patients, however, in 75% (n=3) of the four neurologically symptomatic patients it was bilaterally thickened and bright on long-TR MR images. The bright claustrum appears to be a new sign in Wilson's disease.